diff --git a/src/gmt_api.c b/src/gmt_api.c
index 1dc901e8e1b..12f42136889 100644
--- a/src/gmt_api.c
+++ b/src/gmt_api.c
@@ -1984,7 +1984,6 @@ GMT_LOCAL int gmtapi_init_matrix (struct GMTAPI_CTRL *API, uint64_t dim[], doubl
 	/* If range = inc = NULL then add dimensioning is set via dim: ncols, nrow, nlayers, type.
 	 * else, ncols,nrows is set via range and inc and registration. dim, if not null, sets dim[2] = nlayers [1] and dim[3] = type [double]
 	 */
-	double off = 0.5 * registration;
 	int error;
 	unsigned int dims = (M->n_layers > 1) ? 3 : 2;
 	size_t size = 0;
@@ -1999,8 +1998,8 @@ GMT_LOCAL int gmtapi_init_matrix (struct GMTAPI_CTRL *API, uint64_t dim[], doubl
 			if (!inc || (inc[GMT_X] == 0.0 && inc[GMT_Y] == 0.0)) return (GMT_VALUE_NOT_SET);
 			gmt_M_memcpy (M->range, range, 2 * dims, double);
 			gmt_M_memcpy (M->inc, inc, dims, double);
-			M->n_rows    = gmt_M_get_n (API->GMT, range[YLO], range[YHI], inc[GMT_Y], off);
-			M->n_columns = gmt_M_get_n (API->GMT, range[XLO], range[XHI], inc[GMT_X], off);
+			M->n_rows    = gmt_M_get_n (API->GMT, range[YLO], range[YHI], inc[GMT_Y], registration);
+			M->n_columns = gmt_M_get_n (API->GMT, range[XLO], range[XHI], inc[GMT_X], registration);
 			M->dim = (M->shape == GMT_IS_ROW_FORMAT) ? M->n_columns : M->n_rows;	/* Matrix layout order */
 		}
 		return (GMT_NOERROR);
@@ -2016,11 +2015,12 @@ GMT_LOCAL int gmtapi_init_matrix (struct GMTAPI_CTRL *API, uint64_t dim[], doubl
 		if (!inc || (inc[GMT_X] == 0.0 && inc[GMT_Y] == 0.0)) return (GMT_VALUE_NOT_SET);
 		gmt_M_memcpy (M->range, range, 2 * dims, double);
 		gmt_M_memcpy (M->inc, inc, dims, double);
-		M->n_rows    = gmt_M_get_n (API->GMT, range[YLO], range[YHI], inc[GMT_Y], off);
-		M->n_columns = gmt_M_get_n (API->GMT, range[XLO], range[XHI], inc[GMT_X], off);
+		M->n_rows    = gmt_M_get_n (API->GMT, range[YLO], range[YHI], inc[GMT_Y], registration);
+		M->n_columns = gmt_M_get_n (API->GMT, range[XLO], range[XHI], inc[GMT_X], registration);
 	}
 	M->type = (dim == NULL) ? GMT_DOUBLE : dim[3];	/* Use selected data type for export or default to double */
 	M->dim = (M->shape == GMT_IS_ROW_FORMAT) ? M->n_columns : M->n_rows;
+	M->registration = registration;
 	size = M->n_rows * M->n_columns * ((size_t)M->n_layers);	/* Size in bytes of the initial matrix allocation */
 	if ((mode & GMT_CONTAINER_ONLY) == 0) {	/* Must allocate data memory */
 		struct GMT_MATRIX_HIDDEN *MH = gmt_get_M_hidden (M);
@@ -2042,7 +2042,7 @@ GMT_LOCAL uint64_t gmtapi_vector_nrows (uint64_t dim[], double *range, double *i
 	if (dim && dim[GMTAPI_DIM_ROW]) return dim[GMTAPI_DIM_ROW];	/* Gave the dimension directly */
 	if (dir == GMT_IN && (!inc || inc[GMT_X] == 0.0)) return ((uint64_t)GMT_NOTSET);
 	if (dir == GMT_IN && (!range || (range[XLO] == 0.0 && range[XHI] == 0.0))) return ((uint64_t)GMT_NOTSET);
-	if (range && inc) return (gmt_M_get_n (API->GMT, range[XLO], range[XHI], inc[GMT_X], 0.5 * registration));
+	if (range && inc) return (gmt_M_get_n (API->GMT, range[XLO], range[XHI], inc[GMT_X], registration));
 	return (0);	/* When dir == GMT_OUT */
 }
 
diff --git a/src/testapi_matrix_as_grid.c b/src/testapi_matrix_as_grid.c
new file mode 100644
index 00000000000..2eed6b762d1
--- /dev/null
+++ b/src/testapi_matrix_as_grid.c
@@ -0,0 +1,41 @@
+#include "gmt_dev.h"
+
+/* Test passing a gridline and pixel-registered matrix as grids to grdimage.
+ * the two plots should be identical.  This tests was made to demonstrate a
+ * bug in the API that was fixed in https://github.com/GenericMappingTools/gmt/pull/3503.
+ * The problem was an error in gmt_M_get_n and failure to set M->registration for matrices.
+ */
+
+int main () {
+	unsigned int mode = GMT_SESSION_EXTERNAL;
+	struct GMT_MATRIX *M_p = NULL, *M_g = NULL;
+	char input_p[GMT_VF_LEN] = {""}, input_g[GMT_VF_LEN] = {""};
+	char args_p[128] = {""}, args_g[128]= {""};
+	struct GMTAPI_CTRL *API = NULL;
+
+	double inc[2] = {1.0, 1.0};
+	double coord[9] = {0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9};
+
+	API = GMT_Create_Session ("test", 2U, mode, NULL);
+
+	/* pass matrix as gridline-registered grid */
+	double range_g[4] = {0.0, 2.0, 0.0, 2.0};
+	if ((M_g = GMT_Create_Data (API, GMT_IS_GRID|GMT_VIA_MATRIX, GMT_IS_SURFACE, GMT_CONTAINER_ONLY, NULL, range_g, inc, GMT_GRID_NODE_REG, 0, NULL)) == NULL) return (EXIT_FAILURE);
+	GMT_Put_Matrix (API, M_g, GMT_DOUBLE, 0, coord);
+	GMT_Open_VirtualFile (API, GMT_IS_GRID|GMT_VIA_MATRIX, GMT_IS_SURFACE, GMT_IN|GMT_IS_REFERENCE, M_g, input_g);
+	sprintf (args_g, "%s -R-1/3/-1/3 -JX6c -Baf -BWSen+tGridline -K -P > api_matrix_as_grid.ps", input_g);
+	GMT_Call_Module (API, "grdimage", GMT_MODULE_CMD, args_g);
+	GMT_Close_VirtualFile (API, input_g);
+
+	/* pass matrix as pixel-registered grid */
+	double range_p[4] = {-0.5, 2.5, -0.5, 2.5};
+	if ((M_p = GMT_Create_Data (API, GMT_IS_GRID|GMT_VIA_MATRIX, GMT_IS_SURFACE, GMT_CONTAINER_ONLY, NULL, range_p, inc, GMT_GRID_PIXEL_REG, 0, NULL)) == NULL) return (EXIT_FAILURE);
+	GMT_Put_Matrix (API, M_p, GMT_DOUBLE, 0, coord);
+	GMT_Open_VirtualFile (API, GMT_IS_GRID|GMT_VIA_MATRIX, GMT_IS_SURFACE, GMT_IN|GMT_IS_REFERENCE, M_p, input_p);
+	sprintf (args_p, "%s -R-1/3/-1/3 -JX6c -Baf -BWSen+tPixel -O -X8c >> api_matrix_as_grid.ps", input_p);
+	GMT_Call_Module (API, "grdimage", GMT_MODULE_CMD, args_p);
+	GMT_Close_VirtualFile (API, input_p);
+
+	if (GMT_Destroy_Session (API)) return EXIT_FAILURE;
+	exit (0);
+}
diff --git a/test/api/api_matrix_as_grid.ps b/test/api/api_matrix_as_grid.ps
new file mode 100644
index 00000000000..423919092b3
Binary files /dev/null and b/test/api/api_matrix_as_grid.ps differ
diff --git a/test/api/apigrdpix.sh b/test/api/apigrdpix.sh
new file mode 100755
index 00000000000..591fdbe7aad
--- /dev/null
+++ b/test/api/apigrdpix.sh
@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+#
+# Test the C API for passing matrices to grdimage as grids
+ps=api_matrix_as_grid.ps
+testapi_matrix_as_grid