diff --git a/pygmt/helpers/__init__.py b/pygmt/helpers/__init__.py
index 08583896b6c..0d074061ba1 100644
--- a/pygmt/helpers/__init__.py
+++ b/pygmt/helpers/__init__.py
@@ -19,7 +19,6 @@
     _validate_data_input,
     args_in_kwargs,
     build_arg_list,
-    build_arg_string,
     data_kind,
     is_nonstr_iter,
     launch_external_viewer,
diff --git a/pygmt/helpers/utils.py b/pygmt/helpers/utils.py
index 337c27ffbd5..33ef60b4c98 100644
--- a/pygmt/helpers/utils.py
+++ b/pygmt/helpers/utils.py
@@ -9,7 +9,6 @@
 import subprocess
 import sys
 import time
-import warnings
 import webbrowser
 from collections.abc import Iterable, Sequence
 from typing import Any, Literal
@@ -461,139 +460,6 @@ def build_arg_list(  # noqa: PLR0912
     return gmt_args
 
 
-def build_arg_string(kwdict, confdict=None, infile=None, outfile=None):
-    r"""
-    Convert keyword dictionaries and input/output files into a GMT argument string.
-
-    Make sure all values in ``kwdict`` have been previously converted to a
-    string representation using the ``kwargs_to_strings`` decorator. The only
-    exceptions are True, False and None.
-
-    Any lists or tuples left will be interpreted as multiple entries for the
-    same command line option. For example, the kwargs entry ``'B': ['xa',
-    'yaf']`` will be converted to ``-Bxa -Byaf`` in the argument string.
-
-    Note that spaces `` `` in arguments are converted to the equivalent octal
-    code ``\040``, except in the case of -J (projection) arguments where PROJ4
-    strings (e.g. "+proj=longlat +datum=WGS84") will have their spaces removed.
-    See https://github.com/GenericMappingTools/pygmt/pull/1487 for more info.
-
-    .. deprecated:: 0.12.0
-
-       Use :func:`build_arg_list` instead.
-
-    Parameters
-    ----------
-    kwdict : dict
-        A dictionary containing parsed keyword arguments.
-    confdict : dict
-        A dictionary containing configurable GMT parameters.
-    infile : str or pathlib.Path
-        The input file.
-    outfile : str or pathlib.Path
-        The output file.
-
-    Returns
-    -------
-    args : str
-        The space-delimited argument string with '-' inserted before each
-        keyword, or '--' inserted before GMT configuration key-value pairs.
-        The keyword arguments are sorted alphabetically, followed by GMT
-        configuration key-value pairs, with optional input file at the
-        beginning and optional output file at the end.
-
-    Examples
-    --------
-
-    >>> print(
-    ...     build_arg_string(
-    ...         dict(
-    ...             A=True,
-    ...             B=False,
-    ...             E=200,
-    ...             J="+proj=longlat +datum=WGS84",
-    ...             P="",
-    ...             R="1/2/3/4",
-    ...             X=None,
-    ...             Y=None,
-    ...             Z=0,
-    ...         )
-    ...     )
-    ... )
-    -A -E200 -J+proj=longlat+datum=WGS84 -P -R1/2/3/4 -Z0
-    >>> print(
-    ...     build_arg_string(
-    ...         dict(
-    ...             R="1/2/3/4",
-    ...             J="X4i",
-    ...             B=["xaf", "yaf", "WSen"],
-    ...             I=("1/1p,blue", "2/0.25p,blue"),
-    ...         )
-    ...     )
-    ... )
-    -BWSen -Bxaf -Byaf -I1/1p,blue -I2/0.25p,blue -JX4i -R1/2/3/4
-    >>> print(build_arg_string(dict(R="1/2/3/4", J="X4i", watre=True)))
-    Traceback (most recent call last):
-      ...
-    pygmt.exceptions.GMTInvalidInput: Unrecognized parameter 'watre'.
-    >>> print(
-    ...     build_arg_string(
-    ...         dict(
-    ...             B=["af", "WSne+tBlank Space"],
-    ...             F='+t"Empty  Spaces"',
-    ...             l="'Void Space'",
-    ...         ),
-    ...     )
-    ... )
-    -BWSne+tBlank\040Space -Baf -F+t"Empty\040\040Spaces" -l'Void\040Space'
-    >>> print(
-    ...     build_arg_string(
-    ...         dict(A="0", B=True, C="rainbow"),
-    ...         confdict=dict(FORMAT_DATE_MAP="o dd"),
-    ...         infile="input.txt",
-    ...         outfile="output.txt",
-    ...     )
-    ... )
-    input.txt -A0 -B -Crainbow --FORMAT_DATE_MAP="o dd" ->output.txt
-    """
-    msg = (
-        "Utility function 'build_arg_string()' is deprecated in v0.12.0 and will be "
-        "removed in v0.14.0. Use 'build_arg_list()' instead."
-    )
-    warnings.warn(msg, category=FutureWarning, stacklevel=2)
-
-    gmt_args = []
-    for key in kwdict:
-        if len(key) > 2:  # raise an exception for unrecognized options
-            raise GMTInvalidInput(f"Unrecognized parameter '{key}'.")
-        if kwdict[key] is None or kwdict[key] is False:
-            pass  # Exclude arguments that are None and False
-        elif is_nonstr_iter(kwdict[key]):
-            for value in kwdict[key]:
-                _value = str(value).replace(" ", r"\040")
-                gmt_args.append(rf"-{key}{_value}")
-        elif kwdict[key] is True:
-            gmt_args.append(f"-{key}")
-        else:
-            if key != "J":  # non-projection parameters
-                _value = str(kwdict[key]).replace(" ", r"\040")
-            else:
-                # special handling if key == "J" (projection)
-                # remove any spaces in PROJ4 string
-                _value = str(kwdict[key]).replace(" ", "")
-            gmt_args.append(rf"-{key}{_value}")
-    gmt_args = sorted(gmt_args)
-
-    if confdict:
-        gmt_args.extend(f'--{key}="{value}"' for key, value in confdict.items())
-
-    if infile:
-        gmt_args = [str(infile), *gmt_args]
-    if outfile:
-        gmt_args.append("->" + str(outfile))
-    return non_ascii_to_octal(" ".join(gmt_args))
-
-
 def is_nonstr_iter(value):
     """
     Check if the value is not a string but is iterable (list, tuple, array)