diff --git a/library/bignum_mod_raw.c b/library/bignum_mod_raw.c
index 6c694b0c58..2f49ea2d9e 100644
--- a/library/bignum_mod_raw.c
+++ b/library/bignum_mod_raw.c
@@ -129,7 +129,16 @@ void mbedtls_mpi_mod_raw_sub( mbedtls_mpi_uint *X,
 /* END MERGE SLOT 4 */
 
 /* BEGIN MERGE SLOT 5 */
-
+void mbedtls_mpi_mod_raw_add( mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_uint *B,
+                              const mbedtls_mpi_mod_modulus *N )
+{
+    mbedtls_mpi_uint carry, borrow;
+    carry  = mbedtls_mpi_core_add( X, A, B, N->limbs );
+    borrow = mbedtls_mpi_core_sub( X, X, N->p, N->limbs );
+    (void) mbedtls_mpi_core_add_if( X, N->p, N->limbs, (unsigned) ( carry ^ borrow ) );
+}
 /* END MERGE SLOT 5 */
 
 /* BEGIN MERGE SLOT 6 */
diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index 05fa9d6ac8..f6c6ebd8f2 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -177,7 +177,28 @@ void mbedtls_mpi_mod_raw_sub( mbedtls_mpi_uint *X,
 /* END MERGE SLOT 4 */
 
 /* BEGIN MERGE SLOT 5 */
-
+/**
+ * \brief Perform a known-size modular addition.
+ *
+ * Calculate `A + B modulo N`.
+ *
+ * The number of limbs in each operand, and the result, is given by the
+ * modulus \p N.
+ *
+ * \p X may be aliased to \p A or \p B, or even both, but may not overlap
+ * either otherwise.
+ *
+ * \param[out] X    The result of the modular addition.
+ * \param[in] A     Little-endian presentation of the left operand. This
+ *                  must be smaller than \p N.
+ * \param[in] B     Little-endian presentation of the right operand. This
+ *                  must be smaller than \p N.
+ * \param[in] N     The address of the modulus.
+ */
+void mbedtls_mpi_mod_raw_add( mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_uint *B,
+                              const mbedtls_mpi_mod_modulus *N );
 /* END MERGE SLOT 5 */
 
 /* BEGIN MERGE SLOT 6 */
diff --git a/scripts/mbedtls_dev/bignum_mod_raw.py b/scripts/mbedtls_dev/bignum_mod_raw.py
index c271048548..0bbad5dd90 100644
--- a/scripts/mbedtls_dev/bignum_mod_raw.py
+++ b/scripts/mbedtls_dev/bignum_mod_raw.py
@@ -61,6 +61,19 @@ class BignumModRawSub(bignum_common.ModOperationCommon,
 
 # BEGIN MERGE SLOT 5
 
+class BignumModRawAdd(bignum_common.ModOperationCommon,
+                      BignumModRawTarget):
+    """Test cases for bignum mpi_mod_raw_add()."""
+    symbol = "+"
+    test_function = "mpi_mod_raw_add"
+    test_name = "mbedtls_mpi_mod_raw_add"
+    input_style = "fixed"
+    arity = 2
+
+    def result(self) -> List[str]:
+        result = (self.int_a + self.int_b) % self.int_n
+        return [self.format_result(result)]
+
 # END MERGE SLOT 5
 
 # BEGIN MERGE SLOT 6
diff --git a/tests/suites/test_suite_bignum_mod_raw.function b/tests/suites/test_suite_bignum_mod_raw.function
index 2d9e412280..00ed005f50 100644
--- a/tests/suites/test_suite_bignum_mod_raw.function
+++ b/tests/suites/test_suite_bignum_mod_raw.function
@@ -360,6 +360,98 @@ exit:
 
 /* BEGIN MERGE SLOT 5 */
 
+/* BEGIN_CASE */
+void mpi_mod_raw_add( char * input_N,
+                      char * input_A, char * input_B,
+                      char * input_S )
+{
+    mbedtls_mpi_uint *A = NULL;
+    mbedtls_mpi_uint *B = NULL;
+    mbedtls_mpi_uint *S = NULL;
+    mbedtls_mpi_uint *N = NULL;
+    mbedtls_mpi_uint *X = NULL;
+    size_t A_limbs, B_limbs, N_limbs, S_limbs;
+
+    mbedtls_mpi_mod_modulus m;
+    mbedtls_mpi_mod_modulus_init( &m );
+
+    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &A, &A_limbs, input_A ) );
+    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &B, &B_limbs, input_B ) );
+    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &N, &N_limbs, input_N ) );
+    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &S, &S_limbs, input_S ) );
+
+    /* Modulus gives the number of limbs; all inputs must have the same. */
+    size_t limbs = N_limbs;
+    size_t bytes = limbs * sizeof( *A );
+
+    TEST_EQUAL( A_limbs, limbs );
+    TEST_EQUAL( B_limbs, limbs );
+    TEST_EQUAL( S_limbs, limbs );
+
+    ASSERT_ALLOC( X, limbs );
+
+    TEST_EQUAL( mbedtls_mpi_mod_modulus_setup(
+                        &m, N, limbs,
+                        MBEDTLS_MPI_MOD_EXT_REP_BE,
+                        MBEDTLS_MPI_MOD_REP_MONTGOMERY
+                ), 0 );
+
+    /* A + B => Correct result */
+    mbedtls_mpi_mod_raw_add( X, A, B, &m );
+    ASSERT_COMPARE( X, bytes, S, bytes );
+
+    /* A + B: alias X to A => Correct result */
+    memcpy( X, A, bytes );
+    mbedtls_mpi_mod_raw_add( X, X, B, &m );
+    ASSERT_COMPARE( X, bytes, S, bytes );
+
+    /* A + B: alias X to B => Correct result */
+    memcpy( X, B, bytes );
+    mbedtls_mpi_mod_raw_add( X, A, X, &m );
+    ASSERT_COMPARE( X, bytes, S, bytes );
+
+    if ( memcmp(A, B, bytes ) == 0 )
+    {
+        /* A == B: alias A and B */
+
+        /* A + A => Correct result */
+        mbedtls_mpi_mod_raw_add( X, A, A, &m );
+        ASSERT_COMPARE( X, bytes, S, bytes );
+
+        /* A + A: X, A, B all aliased together => Correct result */
+        memcpy( X, A, bytes );
+        mbedtls_mpi_mod_raw_add( X, X, X, &m );
+        ASSERT_COMPARE( X, bytes, S, bytes );
+    }
+    else
+    {
+        /* A != B: test B + A */
+
+        /* B + A => Correct result */
+        mbedtls_mpi_mod_raw_add( X, B, A, &m );
+        ASSERT_COMPARE( X, bytes, S, bytes );
+
+        /* B + A: alias X to A => Correct result */
+        memcpy( X, A, bytes );
+        mbedtls_mpi_mod_raw_add( X, B, X, &m );
+        ASSERT_COMPARE( X, bytes, S, bytes );
+
+        /* B + A: alias X to B => Correct result */
+        memcpy( X, B, bytes );
+        mbedtls_mpi_mod_raw_add( X, X, A, &m );
+        ASSERT_COMPARE( X, bytes, S, bytes );
+    }
+
+exit:
+    mbedtls_mpi_mod_modulus_free( &m );
+
+    mbedtls_free( A );
+    mbedtls_free( B );
+    mbedtls_free( S );
+    mbedtls_free( N );
+    mbedtls_free( X );
+}
+/* END_CASE */
 /* END MERGE SLOT 5 */
 
 /* BEGIN MERGE SLOT 6 */