Abstract We present a new optical transmission spectrum of the hot Jupiter WASP-79b. We observed three transits with the STIS instrument mounted on the Hubble Space Telescope (HST), spanning 0.3–1.0 μ m. Combining these transits with previous observations, we construct a complete 0.3–5.0 μ m transmission spectrum of WASP-79b. Both HST and ground-based observations show decreasing transit depths toward blue wavelengths, contrary to expectations from Rayleigh scattering or hazes. We infer atmospheric and stellar properties from the full near-UV to infrared transmission spectrum of WASP-79b using three independent retrieval codes, all of which yield consistent results. Our retrievals confirm previous detections of H 2 O (at 4.0 σ confidence) while providing moderate evidence of H − bound–free opacity (3.3 σ ) and strong evidence of stellar contamination from unocculted faculae (4.7 σ ). The retrieved H 2 O abundance (∼1%) suggests a superstellar atmospheric metallicity, though stellar or substellar abundances remain consistent with present observations (O/H = 0.3–34× stellar). All three retrieval codes obtain a precise H − abundance constraint: log( X H − ) ≈ −8.0 ± 0.7. The potential presence of H − suggests that James Webb Space Telescope observations may be sensitive to ionic chemistry in the atmosphere of WASP-79b. The inferred faculae are ∼500 K hotter than the stellar photosphere, covering ∼15% of the stellar surface. Our analysis underscores the importance of observing UV–optical transmission spectra in order to disentangle the influence of unocculted stellar heterogeneities from planetary transmission spectra.