diff --git a/library/bignum_mod.c b/library/bignum_mod.c
index 7cf2fb2..0057eba 100644
--- a/library/bignum_mod.c
+++ b/library/bignum_mod.c
@@ -198,7 +198,18 @@
 /* END MERGE SLOT 4 */
 
 /* BEGIN MERGE SLOT 5 */
+int mbedtls_mpi_mod_add( mbedtls_mpi_mod_residue *X,
+                         const mbedtls_mpi_mod_residue *A,
+                         const mbedtls_mpi_mod_residue *B,
+                         const mbedtls_mpi_mod_modulus *N )
+{
+    if( X->limbs != N->limbs || A->limbs != N->limbs || B->limbs != N->limbs )
+        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
 
+    mbedtls_mpi_mod_raw_add(X->p, A->p, B->p, N);
+
+    return( 0 );
+}
 /* END MERGE SLOT 5 */
 
 /* BEGIN MERGE SLOT 6 */
diff --git a/library/bignum_mod.h b/library/bignum_mod.h
index 0a8f4d3..11b4e98 100644
--- a/library/bignum_mod.h
+++ b/library/bignum_mod.h
@@ -199,7 +199,36 @@
 /* END MERGE SLOT 4 */
 
 /* BEGIN MERGE SLOT 5 */
-
+/**
+ * \brief Perform a fixed-size modular addition.
+ *
+ * Calculate `A + B modulo N`.
+ *
+ * \p A, \p B and \p X must all be associated with the modulus \p N and must
+ * all have the same number of limbs as \p N.
+ *
+ * \p X may be aliased to \p A or \p B, or even both, but may not overlap
+ * either otherwise.
+ *
+ * \note This function does not check that \p A or \p B are in canonical
+ *       form (that is, are < \p N) - that will have been done by
+ *       mbedtls_mpi_mod_residue_setup().
+ *
+ * \param[out] X    The address of the result residue. Must be initialized.
+ *                  Must have the same number of limbs as the modulus \p N.
+ * \param[in]  A    The address of the first input residue.
+ * \param[in]  B    The address of the second input residue.
+ * \param[in]  N    The address of the modulus. Used to perform a modulo
+ *                  operation on the result of the addition.
+ *
+ * \return          \c 0 if successful.
+ * \return          #MBEDTLS_ERR_MPI_BAD_INPUT_DATA if the given MPIs do not
+ *                  have the correct number of limbs.
+ */
+int mbedtls_mpi_mod_add( mbedtls_mpi_mod_residue *X,
+                         const mbedtls_mpi_mod_residue *A,
+                         const mbedtls_mpi_mod_residue *B,
+                         const mbedtls_mpi_mod_modulus *N );
 /* END MERGE SLOT 5 */
 
 /* BEGIN MERGE SLOT 6 */
diff --git a/scripts/mbedtls_dev/bignum_mod.py b/scripts/mbedtls_dev/bignum_mod.py
index aa06fe8..a16699a 100644
--- a/scripts/mbedtls_dev/bignum_mod.py
+++ b/scripts/mbedtls_dev/bignum_mod.py
@@ -14,10 +14,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Dict, List # pylint: disable=unused-import
+from typing import Dict, List
 
 from . import test_data_generation
-from . import bignum_common # pylint: disable=unused-import
+from . import bignum_common
 
 class BignumModTarget(test_data_generation.BaseTarget):
     #pylint: disable=abstract-method, too-few-public-methods
@@ -55,6 +55,20 @@
 # END MERGE SLOT 4
 
 # BEGIN MERGE SLOT 5
+class BignumModAdd(bignum_common.ModOperationCommon, BignumModTarget):
+    """Test cases for bignum mpi_mod_add()."""
+    count = 0
+    symbol = "+"
+    test_function = "mpi_mod_add"
+    test_name = "mbedtls_mpi_mod_add"
+    input_style = "fixed"
+
+    def result(self) -> List[str]:
+        result = (self.int_a + self.int_b) % self.int_n
+        # To make negative tests easier, append "0" for success to the
+        # generated cases
+        return [self.format_result(result), "0"]
+
 
 # END MERGE SLOT 5
 
diff --git a/tests/suites/test_suite_bignum_mod.function b/tests/suites/test_suite_bignum_mod.function
index 0d2e232..507920a 100644
--- a/tests/suites/test_suite_bignum_mod.function
+++ b/tests/suites/test_suite_bignum_mod.function
@@ -210,7 +210,110 @@
 /* END MERGE SLOT 4 */
 
 /* BEGIN MERGE SLOT 5 */
+/* BEGIN_CASE */
+void mpi_mod_add( char * input_N,
+                  char * input_A, char * input_B,
+                  char * input_S, int expected_ret )
+{
+    mbedtls_mpi_mod_residue a = { NULL, 0 };
+    mbedtls_mpi_mod_residue b = { NULL, 0 };
+    mbedtls_mpi_mod_residue s = { NULL, 0 };
+    mbedtls_mpi_mod_residue x = { NULL, 0 };
+    mbedtls_mpi_uint *X_raw = NULL;
 
+    mbedtls_mpi_mod_modulus m;
+    mbedtls_mpi_mod_modulus_init( &m );
+
+    TEST_EQUAL( 0,
+        test_read_modulus( &m, MBEDTLS_MPI_MOD_REP_MONTGOMERY, input_N ) );
+
+    /* test_read_residue() normally checks that inputs have the same number of
+     * limbs as the modulus. For negative testing we can ask it to skip this
+     * with a non-zero final parameter. */
+    TEST_EQUAL( 0, test_read_residue( &a, &m, input_A, expected_ret != 0 ) );
+    TEST_EQUAL( 0, test_read_residue( &b, &m, input_B, expected_ret != 0 ) );
+    TEST_EQUAL( 0, test_read_residue( &s, &m, input_S, expected_ret != 0 ) );
+
+    size_t limbs = m.limbs;
+    size_t bytes = limbs * sizeof( *X_raw );
+
+    if( expected_ret == 0 )
+    {
+        /* Negative test with too many limbs in output */
+        ASSERT_ALLOC( X_raw, limbs + 1 );
+
+        x.p = X_raw;
+        x.limbs = limbs + 1;
+        TEST_EQUAL( MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
+                    mbedtls_mpi_mod_add( &x, &a, &b, &m ) );
+
+        mbedtls_free( X_raw );
+        X_raw = NULL;
+
+        /* Negative test with too few limbs in output */
+        if( limbs > 1 )
+        {
+            ASSERT_ALLOC( X_raw, limbs - 1 );
+
+            x.p = X_raw;
+            x.limbs = limbs - 1;
+            TEST_EQUAL( MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
+                        mbedtls_mpi_mod_add( &x, &a, &b, &m ) );
+
+            mbedtls_free( X_raw );
+            X_raw = NULL;
+        }
+
+        /* Negative testing with too many/too few limbs in a and b is covered by
+         * manually-written test cases with oret != 0. */
+    }
+
+    /* Allocate correct number of limbs for X_raw */
+    ASSERT_ALLOC( X_raw, limbs );
+
+    TEST_EQUAL( 0, mbedtls_mpi_mod_residue_setup( &x, &m, X_raw, limbs ) );
+
+    /* A + B => Correct result or expected error */
+    TEST_EQUAL( expected_ret, mbedtls_mpi_mod_add( &x, &a, &b, &m ) );
+    if( expected_ret != 0 )
+        goto exit;
+
+    TEST_COMPARE_MPI_RESIDUES( x, s );
+
+    /* a + b: alias x to a => Correct result */
+    memcpy( x.p, a.p, bytes );
+    TEST_EQUAL( 0, mbedtls_mpi_mod_add( &x, &x, &b, &m ) );
+    TEST_COMPARE_MPI_RESIDUES( x, s );
+
+    /* a + b: alias x to b => Correct result */
+    memcpy( x.p, b.p, bytes );
+    TEST_EQUAL( 0, mbedtls_mpi_mod_add( &x, &a, &x, &m ) );
+    TEST_COMPARE_MPI_RESIDUES( x, s );
+
+    if ( memcmp( a.p, b.p, bytes ) == 0 )
+    {
+        /* a == b: alias a and b */
+
+        /* a + a => Correct result */
+        TEST_EQUAL( 0, mbedtls_mpi_mod_add( &x, &a, &a, &m ) );
+        TEST_COMPARE_MPI_RESIDUES( x, s );
+
+        /* a + a: x, a, b all aliased together => Correct result */
+        memcpy( x.p, a.p, bytes );
+        TEST_EQUAL( 0, mbedtls_mpi_mod_add( &x, &x, &x, &m ) );
+        TEST_COMPARE_MPI_RESIDUES( x, s );
+    }
+
+exit:
+    mbedtls_free( (void *)m.p ); /* mbedtls_mpi_mod_modulus_free() sets m.p = NULL */
+    mbedtls_mpi_mod_modulus_free( &m );
+
+    mbedtls_free( a.p );
+    mbedtls_free( b.p );
+    mbedtls_free( s.p );
+    mbedtls_free( X_raw );
+}
+/* END_CASE */
 /* END MERGE SLOT 5 */
 
 /* BEGIN MERGE SLOT 6 */
diff --git a/tests/suites/test_suite_bignum_mod.data b/tests/suites/test_suite_bignum_mod.misc.data
similarity index 97%
rename from tests/suites/test_suite_bignum_mod.data
rename to tests/suites/test_suite_bignum_mod.misc.data
index 501d9d7..7b1c85f 100644
--- a/tests/suites/test_suite_bignum_mod.data
+++ b/tests/suites/test_suite_bignum_mod.misc.data
@@ -45,7 +45,26 @@
 # END MERGE SLOT 4
 
 # BEGIN MERGE SLOT 5
+mpi_mod_add base case for negative testing (N, a, b all >= 1 limb)
+mpi_mod_add:"014320a022ccb75bdf470ddf25":"000000025a55a46e5da99c71c7":"00033b2e3c9fd0803ce8000f93":"00033b3096f574ee9a919c815a":0
 
+mpi_mod_add with modulus too long/both inputs too short
+mpi_mod_add:"0000000014320a022ccb75bdf470ddf25":"000000025a55a46e5da99c71c7":"00033b2e3c9fd0803ce8000f93":"00":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
+
+mpi_mod_add with first input too long
+mpi_mod_add:"014320a022ccb75bdf470ddf25":"0000000000000025a55a46e5da99c71c7":"00033b2e3c9fd0803ce8000f93":"00":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
+
+mpi_mod_add with second input too long
+mpi_mod_add:"014320a022ccb75bdf470ddf25":"000000025a55a46e5da99c71c7":"000000000033b2e3c9fd0803ce8000f93":"00":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
+
+mpi_mod_add with both inputs too long
+mpi_mod_add:"014320a022ccb75bdf470ddf25":"0000000000000025a55a46e5da99c71c7":"000000000033b2e3c9fd0803ce8000f93":"00":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
+
+mpi_mod_add with first input too short
+mpi_mod_add:"014320a022ccb75bdf470ddf25":"a99c71c7":"00033b2e3c9fd0803ce8000f93":"00":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
+
+mpi_mod_add with second input too short
+mpi_mod_add:"014320a022ccb75bdf470ddf25":"000000025a55a46e5da99c71c7":"e8000f93":"00":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
 # END MERGE SLOT 5
 
 # BEGIN MERGE SLOT 6