Transverse translation-diverse phase retrieval (TTDPR), a ptychographic wavefront-sensing technique, is a viable method for freeform optical surface metrology due to its relatively simple hardware requirements, flexibility, and demonstrated accuracy in other fields. In TTDPR, a subaperture illumination pattern is scanned across an optic under test, and the reflected intensity is gathered on an array detector near focus. A nonlinear optimization algorithm is used to reconstruct the wavefront aberration at the test surface from which we can solve for surface error, using intensity patterns from multiple scan positions. TTDPR is an advantageous method for aspheric and freeform metrology because measurements can be performed without null optics. We report on the development of a concave freeform mirror measurement using this technique. Simulations were performed to test algorithmic performance as a function of various parameters, including detector signal-to-noise ratio and position uncertainty of the illumination, with <λ  /  100 root-mean-square (rms) wavefront-sensing error achieved in most cases (λ  =  632.8  nm). An experimental measurement is then demonstrated, and results of reconstructed surface form and midspatial frequency error are presented. Surface reconstructions from two disjoint datasets agree to 13-nm rms, or λ  /  50 at λ  =  632.8  nm.