Mechanical constitutive models of metals undergoing deformation in three dimensions can be challenging to validate without loss of generality. For example, creep-induced stress relaxation in stainless steels has previously been studied using axisymmetric notched-bar tests which control the triaxiality of the initial stress state. However, such experiments only provide partial insight into the creep process that causes the specimen’s stress state to vary spatially and over time. We have used time-of-flight neutron diffraction to track the complete stress tensor at 12 interior locations within specimens of Type 316H stainless steel containing a complex stress field as it relaxes due to creep. Using such data, it is possible to check the accuracy of creep laws (such as the widely-used RCC-MR model) in the general multiaxial case. Over-determination of the elastic strain tensor using measurements in multiple directions also helps to reduce measurement uncertainty. Our results indicate that the RCC-MR primary/secondary creep law for Type 316H is conservative for cases involving a complex initial stress field.