diff --git a/ChangeLog.d/mpi-most-negative-sint.txt b/ChangeLog.d/mpi-most-negative-sint.txt
new file mode 100644
index 0000000..5e775c4
--- /dev/null
+++ b/ChangeLog.d/mpi-most-negative-sint.txt
@@ -0,0 +1,4 @@
+Bugfix
+   * Fix undefined behavior (typically harmless in practice) when some bignum
+     functions receive the most negative value of mbedtls_mpi_sint. Credit
+     to OSS-Fuzz. Fixes #6597.
diff --git a/library/bignum.c b/library/bignum.c
index 497ccbc..04aca69 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -263,7 +263,7 @@
     MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, 1 ) );
     memset( X->p, 0, X->n * ciL );
 
-    X->p[0] = ( z < 0 ) ? -z : z;
+    X->p[0] = ( z < 0 ) ? -(mbedtls_mpi_uint)z : z;
     X->s    = ( z < 0 ) ? -1 : 1;
 
 cleanup:
@@ -853,7 +853,7 @@
     mbedtls_mpi_uint p[1];
     MPI_VALIDATE_RET( X != NULL );
 
-    *p  = ( z < 0 ) ? -z : z;
+    *p  = ( z < 0 ) ? -(mbedtls_mpi_uint)z : z;
     Y.s = ( z < 0 ) ? -1 : 1;
     Y.n = 1;
     Y.p = p;
@@ -1057,7 +1057,7 @@
     MPI_VALIDATE_RET( X != NULL );
     MPI_VALIDATE_RET( A != NULL );
 
-    p[0] = ( b < 0 ) ? -b : b;
+    p[0] = ( b < 0 ) ? -(mbedtls_mpi_uint)b : b;
     B.s = ( b < 0 ) ? -1 : 1;
     B.n = 1;
     B.p = p;
@@ -1075,7 +1075,7 @@
     MPI_VALIDATE_RET( X != NULL );
     MPI_VALIDATE_RET( A != NULL );
 
-    p[0] = ( b < 0 ) ? -b : b;
+    p[0] = ( b < 0 ) ? -(mbedtls_mpi_uint)b : b;
     B.s = ( b < 0 ) ? -1 : 1;
     B.n = 1;
     B.p = p;
@@ -1413,7 +1413,7 @@
     mbedtls_mpi_uint p[1];
     MPI_VALIDATE_RET( A != NULL );
 
-    p[0] = ( b < 0 ) ? -b : b;
+    p[0] = ( b < 0 ) ? -(mbedtls_mpi_uint)b : b;
     B.s = ( b < 0 ) ? -1 : 1;
     B.n = 1;
     B.p = p;
diff --git a/tests/suites/test_suite_bignum.function b/tests/suites/test_suite_bignum.function
index 5c3d776..3238467 100644
--- a/tests/suites/test_suite_bignum.function
+++ b/tests/suites/test_suite_bignum.function
@@ -1447,6 +1447,150 @@
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void most_negative_mpi_sint( )
+{
+    /* Ad hoc tests for n = -p = -2^(biL-1) as a mbedtls_mpi_sint. We
+     * guarantee that mbedtls_mpi_sint is a two's complement type, so this
+     * is a valid value. However, negating it (`-n`) has undefined behavior
+     * (although in practice `-n` evaluates to the value n).
+     *
+     * This function has ad hoc tests for this value. It's separated from other
+     * functions because the test framework makes it hard to pass this value
+     * into test cases.
+     *
+     * In the comments here:
+     * - biL = number of bits in limbs
+     * - p = 2^(biL-1) (smallest positive value not in mbedtls_mpi_sint range)
+     * - n = -2^(biL-1) (largest negative value in mbedtls_mpi_sint range)
+     */
+
+    mbedtls_mpi A, R, X;
+    mbedtls_mpi_init( &A );
+    mbedtls_mpi_init( &R );
+    mbedtls_mpi_init( &X );
+
+    const size_t biL = 8 * sizeof( mbedtls_mpi_sint );
+    mbedtls_mpi_uint most_positive_plus_1 = (mbedtls_mpi_uint) 1 << ( biL - 1 );
+    const mbedtls_mpi_sint most_positive = most_positive_plus_1 - 1;
+    const mbedtls_mpi_sint most_negative = - most_positive - 1;
+    TEST_EQUAL( (mbedtls_mpi_uint) most_negative,
+                (mbedtls_mpi_uint) 1 << ( biL - 1 ) );
+    TEST_EQUAL( (mbedtls_mpi_uint) most_negative << 1, 0 );
+
+    /* Test mbedtls_mpi_lset() */
+    TEST_EQUAL( mbedtls_mpi_lset( &A, most_negative ), 0 );
+    TEST_EQUAL( A.s, -1 );
+    TEST_EQUAL( A.n, 1 );
+    TEST_EQUAL( A.p[0], most_positive_plus_1 );
+
+    /* Test mbedtls_mpi_cmp_int(): -p == -p */
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &A, most_negative ), 0 );
+
+    /* Test mbedtls_mpi_cmp_int(): -(p+1) < -p */
+    A.p[0] = most_positive_plus_1 + 1;
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &A, most_negative ), -1 );
+
+    /* Test mbedtls_mpi_cmp_int(): -(p-1) > -p */
+    A.p[0] = most_positive_plus_1 - 1;
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &A, most_negative ), 1 );
+
+    /* Test mbedtls_mpi_add_int(): (p-1) + (-p) */
+    TEST_EQUAL( mbedtls_mpi_lset( &A, most_positive ), 0 );
+    TEST_EQUAL( mbedtls_mpi_add_int( &X, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, -1 ), 0 );
+
+    /* Test mbedtls_mpi_add_int(): (0) + (-p) */
+    TEST_EQUAL( mbedtls_mpi_lset( &A, 0 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_add_int( &X, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, most_negative ), 0 );
+
+    /* Test mbedtls_mpi_add_int(): (-p) + (-p) */
+    TEST_EQUAL( mbedtls_mpi_lset( &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_add_int( &X, &A, most_negative ), 0 );
+    TEST_EQUAL( X.s, -1 );
+    TEST_EQUAL( X.n, 2 );
+    TEST_EQUAL( X.p[0], 0 );
+    TEST_EQUAL( X.p[1], 1 );
+
+    /* Test mbedtls_mpi_sub_int(): (p) - (-p) */
+    mbedtls_mpi_free( &X );
+    TEST_EQUAL( mbedtls_mpi_lset( &A, most_positive ), 0 );
+    TEST_EQUAL( mbedtls_mpi_sub_int( &X, &A, most_negative ), 0 );
+    TEST_EQUAL( X.s, 1 );
+    TEST_EQUAL( X.n, 1 );
+    TEST_EQUAL( X.p[0], ~(mbedtls_mpi_uint)0 );
+
+    /* Test mbedtls_mpi_sub_int(): (0) - (-p) */
+    TEST_EQUAL( mbedtls_mpi_lset( &A, 0 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_sub_int( &X, &A, most_negative ), 0 );
+    TEST_EQUAL( X.s, 1 );
+    TEST_EQUAL( X.n, 1 );
+    TEST_EQUAL( X.p[0], most_positive_plus_1 );
+
+    /* Test mbedtls_mpi_sub_int(): (-p) - (-p) */
+    TEST_EQUAL( mbedtls_mpi_lset( &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_sub_int( &X, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 0 ), 0 );
+
+    /* Test mbedtls_mpi_div_int(): (-p+1) / (-p) */
+    TEST_EQUAL( mbedtls_mpi_lset( &A, -most_positive ), 0 );
+    TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 0 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &R, -most_positive ), 0 );
+
+    /* Test mbedtls_mpi_div_int(): (-p) / (-p) */
+    TEST_EQUAL( mbedtls_mpi_lset( &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 1 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &R, 0 ), 0 );
+
+    /* Test mbedtls_mpi_div_int(): (-2*p) / (-p) */
+    TEST_EQUAL( mbedtls_mpi_shift_l( &A, 1 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 2 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &R, 0 ), 0 );
+
+    /* Test mbedtls_mpi_div_int(): (-2*p+1) / (-p) */
+    TEST_EQUAL( mbedtls_mpi_add_int( &A, &A, 1 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 1 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &R, -most_positive ), 0 );
+
+    /* Test mbedtls_mpi_div_int(): (p-1) / (-p) */
+    TEST_EQUAL( mbedtls_mpi_lset( &A, most_positive ), 0 );
+    TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 0 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &R, most_positive ), 0 );
+
+    /* Test mbedtls_mpi_div_int(): (p) / (-p) */
+    TEST_EQUAL( mbedtls_mpi_add_int( &A, &A, 1 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, -1 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &R, 0 ), 0 );
+
+    /* Test mbedtls_mpi_div_int(): (2*p) / (-p) */
+    TEST_EQUAL( mbedtls_mpi_shift_l( &A, 1 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &X, -2 ), 0 );
+    TEST_EQUAL( mbedtls_mpi_cmp_int( &R, 0 ), 0 );
+
+    /* Test mbedtls_mpi_mod_int(): never valid */
+    TEST_EQUAL( mbedtls_mpi_mod_int( X.p, &A, most_negative ),
+                MBEDTLS_ERR_MPI_NEGATIVE_VALUE );
+
+    /* Test mbedtls_mpi_random(): never valid */
+    TEST_EQUAL( mbedtls_mpi_random( &X, most_negative, &A,
+                                    mbedtls_test_rnd_std_rand, NULL ),
+                MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+
+exit:
+    mbedtls_mpi_free( &A );
+    mbedtls_mpi_free( &R );
+    mbedtls_mpi_free( &X );
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_SELF_TEST */
 void mpi_selftest(  )
 {
diff --git a/tests/suites/test_suite_bignum.misc.data b/tests/suites/test_suite_bignum.misc.data
index 0b8aa33..7aaacbe 100644
--- a/tests/suites/test_suite_bignum.misc.data
+++ b/tests/suites/test_suite_bignum.misc.data
@@ -1934,6 +1934,9 @@
 MPI random bad arguments: min > N = 1, 0 limb in upper bound
 mpi_random_fail:2:"000000000000000001":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
 
+Most negative mbedtls_mpi_sint
+most_negative_mpi_sint:
+
 MPI Selftest
 depends_on:MBEDTLS_SELF_TEST
 mpi_selftest: