diff --git a/pygmt/helpers/decorators.py b/pygmt/helpers/decorators.py
index a0ec24e2848..36fa194f904 100644
--- a/pygmt/helpers/decorators.py
+++ b/pygmt/helpers/decorators.py
@@ -269,7 +269,7 @@
             the viewpoint [Default is ``[180, 90]``]. Full documentation is at
             :gmt-docs:`gmt.html#perspective-full`.""",
     "projection": r"""
-        projection : str
+        projection
             *projcode*\[*projparams*/]\ *width*\|\ *scale*.
             Select map :doc:`projection </projections/index>`.""",
     "region": r"""
@@ -415,7 +415,7 @@ def fmt_docstring(module_func):
     region : str or list
         *xmin/xmax/ymin/ymax*\ [**+r**][**+u**\ *unit*].
         Specify the :doc:`region </tutorials/basics/regions>` of interest.
-    projection : str
+    projection
         *projcode*\[*projparams*/]\ *width*\|\ *scale*.
         Select map :doc:`projection </projections/index>`.
     <BLANKLINE>
diff --git a/pygmt/src/basemap.py b/pygmt/src/basemap.py
index 167d3e11ecf..b7753893319 100644
--- a/pygmt/src/basemap.py
+++ b/pygmt/src/basemap.py
@@ -25,7 +25,7 @@
 @kwargs_to_strings(R="sequence", p="sequence")
 def basemap(
     self,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/coast.py b/pygmt/src/coast.py
index 3c5bfef0719..5a82482a8aa 100644
--- a/pygmt/src/coast.py
+++ b/pygmt/src/coast.py
@@ -37,7 +37,7 @@
 @kwargs_to_strings(R="sequence", p="sequence")
 def coast(
     self,
-    projection=None,
+    projection: str | None = None,
     resolution: Literal[
         "auto", "full", "high", "intermediate", "low", "crude", None
     ] = None,
diff --git a/pygmt/src/colorbar.py b/pygmt/src/colorbar.py
index c619712c7bc..857f0712e8e 100644
--- a/pygmt/src/colorbar.py
+++ b/pygmt/src/colorbar.py
@@ -29,7 +29,7 @@
 @kwargs_to_strings(R="sequence", G="sequence", I="sequence", p="sequence")
 def colorbar(
     self,
-    projection=None,
+    projection: str | None = None,
     box: Box | bool = False,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
diff --git a/pygmt/src/contour.py b/pygmt/src/contour.py
index 154cb7830bc..fc08179d79e 100644
--- a/pygmt/src/contour.py
+++ b/pygmt/src/contour.py
@@ -43,7 +43,7 @@ def contour(
     y=None,
     z=None,
     no_clip: bool = False,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/grdcontour.py b/pygmt/src/grdcontour.py
index 70399af5c8b..d363bf15d27 100644
--- a/pygmt/src/grdcontour.py
+++ b/pygmt/src/grdcontour.py
@@ -40,7 +40,7 @@
 def grdcontour(
     self,
     grid: PathLike | xr.DataArray,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/grdcut.py b/pygmt/src/grdcut.py
index d38d2aac8ab..4ae0ff0ff01 100644
--- a/pygmt/src/grdcut.py
+++ b/pygmt/src/grdcut.py
@@ -33,7 +33,7 @@ def grdcut(
     grid: PathLike | xr.DataArray,
     kind: Literal["grid", "image"] = "grid",
     outgrid: PathLike | None = None,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     **kwargs,
diff --git a/pygmt/src/grdimage.py b/pygmt/src/grdimage.py
index ca131b84e49..d27d5472252 100644
--- a/pygmt/src/grdimage.py
+++ b/pygmt/src/grdimage.py
@@ -33,7 +33,7 @@ def grdimage(
     grid: PathLike | xr.DataArray,
     monochrome: bool = False,
     no_clip: bool = False,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/grdproject.py b/pygmt/src/grdproject.py
index 81456d9477d..afd8e4e975a 100644
--- a/pygmt/src/grdproject.py
+++ b/pygmt/src/grdproject.py
@@ -30,7 +30,7 @@
 def grdproject(
     grid: PathLike | xr.DataArray,
     outgrid: PathLike | None = None,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     **kwargs,
diff --git a/pygmt/src/grdview.py b/pygmt/src/grdview.py
index 6fb791d5572..52268299e52 100644
--- a/pygmt/src/grdview.py
+++ b/pygmt/src/grdview.py
@@ -35,7 +35,7 @@
 def grdview(
     self,
     grid: PathLike | xr.DataArray,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/histogram.py b/pygmt/src/histogram.py
index 0996a006b9c..357d1b55391 100644
--- a/pygmt/src/histogram.py
+++ b/pygmt/src/histogram.py
@@ -40,7 +40,7 @@
 def histogram(
     self,
     data: PathLike | TableLike,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/image.py b/pygmt/src/image.py
index db35c1af5c7..1ef00b887c7 100644
--- a/pygmt/src/image.py
+++ b/pygmt/src/image.py
@@ -17,7 +17,7 @@
 def image(
     self,
     imagefile: PathLike,
-    projection=None,
+    projection: str | None = None,
     box: Box | bool = False,
     monochrome: bool = False,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
diff --git a/pygmt/src/inset.py b/pygmt/src/inset.py
index f59153be79f..617bb36a274 100644
--- a/pygmt/src/inset.py
+++ b/pygmt/src/inset.py
@@ -19,7 +19,7 @@
 @kwargs_to_strings(D="sequence", M="sequence", R="sequence")
 def inset(
     self,
-    projection=None,
+    projection: str | None = None,
     box: Box | bool = False,
     no_clip: bool = False,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
diff --git a/pygmt/src/legend.py b/pygmt/src/legend.py
index 9e609197559..8e299bf401c 100644
--- a/pygmt/src/legend.py
+++ b/pygmt/src/legend.py
@@ -26,7 +26,7 @@
 def legend(
     self,
     spec: PathLike | io.StringIO | None = None,
-    projection=None,
+    projection: str | None = None,
     position="JTR+jTR+o0.2c",
     box: Box | bool = False,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
diff --git a/pygmt/src/logo.py b/pygmt/src/logo.py
index d6bbdff2d09..c74ee2dea2b 100644
--- a/pygmt/src/logo.py
+++ b/pygmt/src/logo.py
@@ -15,7 +15,7 @@
 @kwargs_to_strings(R="sequence", p="sequence")
 def logo(
     self,
-    projection=None,
+    projection: str | None = None,
     style: Literal["standard", "url", "no_label"] = "standard",
     box: Box | bool = False,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
diff --git a/pygmt/src/meca.py b/pygmt/src/meca.py
index cc6b8db85ef..335496d0acf 100644
--- a/pygmt/src/meca.py
+++ b/pygmt/src/meca.py
@@ -143,7 +143,7 @@ def meca(  # noqa: PLR0913
     plot_latitude: float | Sequence[float] | None = None,
     event_name: str | Sequence[str] | None = None,
     no_clip: bool = False,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/plot.py b/pygmt/src/plot.py
index 5c84782b9b8..1f54088c6e4 100644
--- a/pygmt/src/plot.py
+++ b/pygmt/src/plot.py
@@ -57,7 +57,7 @@ def plot(  # noqa: PLR0912, PLR0913
     symbol=None,
     direction=None,
     straight_line: bool | Literal["x", "y"] = False,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/plot3d.py b/pygmt/src/plot3d.py
index 61b211119cc..cc3e123a345 100644
--- a/pygmt/src/plot3d.py
+++ b/pygmt/src/plot3d.py
@@ -59,7 +59,7 @@ def plot3d(  # noqa: PLR0912, PLR0913
     symbol=None,
     direction=None,
     straight_line: bool | Literal["x", "y"] = False,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/select.py b/pygmt/src/select.py
index d894d8dd771..15fbcd1d79d 100644
--- a/pygmt/src/select.py
+++ b/pygmt/src/select.py
@@ -50,7 +50,7 @@ def select(
     resolution: Literal[
         "auto", "full", "high", "intermediate", "low", "crude", None
     ] = None,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     **kwargs,
diff --git a/pygmt/src/solar.py b/pygmt/src/solar.py
index 372e168d0c7..6641961d0b8 100644
--- a/pygmt/src/solar.py
+++ b/pygmt/src/solar.py
@@ -22,7 +22,7 @@ def solar(
     terminator_datetime=None,
     fill: str | None = None,
     pen: str | None = None,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/subplot.py b/pygmt/src/subplot.py
index 9829c52a6b2..f80d8a2f538 100644
--- a/pygmt/src/subplot.py
+++ b/pygmt/src/subplot.py
@@ -35,7 +35,7 @@ def subplot(
     self,
     nrows=1,
     ncols=1,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     **kwargs,
diff --git a/pygmt/src/text.py b/pygmt/src/text.py
index baa4694a2a3..5ed30087c56 100644
--- a/pygmt/src/text.py
+++ b/pygmt/src/text.py
@@ -50,7 +50,7 @@ def text_(  # noqa: PLR0912, PLR0913, PLR0915
     font=None,
     justify: bool | None | AnchorCode | Sequence[AnchorCode] = None,
     no_clip: bool = False,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/tilemap.py b/pygmt/src/tilemap.py
index 6f56ed19972..e6afb6ae0c1 100644
--- a/pygmt/src/tilemap.py
+++ b/pygmt/src/tilemap.py
@@ -37,7 +37,7 @@ def tilemap(  # noqa: PLR0913
     zoom_adjust: int | None = None,
     monochrome: bool = False,
     no_clip: bool = False,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/triangulate.py b/pygmt/src/triangulate.py
index 5eb1eb501bf..d0f67488b6a 100644
--- a/pygmt/src/triangulate.py
+++ b/pygmt/src/triangulate.py
@@ -70,7 +70,7 @@ def regular_grid(
         y=None,
         z=None,
         outgrid: PathLike | None = None,
-        projection=None,
+        projection: str | None = None,
         verbose: Literal[
             "quiet", "error", "warning", "timing", "info", "compat", "debug"
         ]
@@ -190,7 +190,7 @@ def delaunay_triples(
         *,
         output_type: Literal["pandas", "numpy", "file"] = "pandas",
         outfile: PathLike | None = None,
-        projection=None,
+        projection: str | None = None,
         verbose: Literal[
             "quiet", "error", "warning", "timing", "info", "compat", "debug"
         ]
diff --git a/pygmt/src/velo.py b/pygmt/src/velo.py
index 6c92b063cb5..56e4f46559e 100644
--- a/pygmt/src/velo.py
+++ b/pygmt/src/velo.py
@@ -39,7 +39,7 @@ def velo(
     self,
     data: PathLike | TableLike | None = None,
     no_clip: bool = False,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/wiggle.py b/pygmt/src/wiggle.py
index fdb86333229..57521001f97 100644
--- a/pygmt/src/wiggle.py
+++ b/pygmt/src/wiggle.py
@@ -64,7 +64,7 @@ def wiggle(  # noqa: PLR0913
     z=None,
     fillpositive=None,
     fillnegative=None,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     panel: int | tuple[int, int] | bool = False,
diff --git a/pygmt/src/xyz2grd.py b/pygmt/src/xyz2grd.py
index 3103a8f6f5b..15a0fa7920b 100644
--- a/pygmt/src/xyz2grd.py
+++ b/pygmt/src/xyz2grd.py
@@ -36,7 +36,7 @@ def xyz2grd(
     y=None,
     z=None,
     outgrid: PathLike | None = None,
-    projection=None,
+    projection: str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
     | bool = False,
     **kwargs,