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We have detected a strongly lensed supernova (SN) in HST imaging of the RX J2129 galaxy-cluster field acquired on August 7 comma 2022. The SN.s apparent F110W magnitude of 24.70+-0.15 and a non-detection in F606W yield an 80% probability that the transient is a Type Ia SN. The foreground galaxy-cluster gravitational lens creates a set of three images of the SN.s host galaxy at redshift 1.52 comma and we have detected the SN in the trailing comma or last-arriving image. The other images are predicted to have arrived virgul320 and virgul1000 days previously. A target-of-opportunity program with HST has been triggered to measure the transient.s light curve through late October. Here we propose three single-orbit WFC3 IR F110W-F160W visits followed by a two-orbit visit to measure the SN.s light curve through late December. In a joint proposal comma we request NIRSpec G140M and PRISM spectroscopy at the locations of all three images in early October comma when the field becomes accessible to JWST. If the SN is a Type Ia SN comma then we will be able to calibrate its luminosity and directly measure its magnification. This will provide a novel test of galaxy-cluster mass models. The spectra will enable comparision with low-redshift SNe Ia to address an important systematic that may affect SN Ia cosmology. We also propose for deep JWST NIRCam imaging to measure the flux of the previous arrival of the SN. With 5-sigma limiting depth of 29.2 comma imaging in F150W can be expected to detect the fading image. Its brightness will constrain the SN.s age comma and therefore the image.s relative time delay. A strong detection and well-characterized SN Ia may enable a new comma independent measurement of the Hubble constant.