We present a Hubble Space Telescope Wide Field Camera-3 (WFC3) transmission spectrum for the transiting exoplanet HAT-P-12b. This warm (1000 K) sub-Saturn-mass planet has a smaller mass and a lower temperature than the hot Jupiters that have been studied so far. We find that the planet's measured transmission spectrum lacks the expected water absorption feature for a hydrogen-dominated atmosphere and is instead best described by a model with high-altitude clouds. Using a frequentist hypothesis

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... esting procedure, we can rule out a hydrogen-dominated cloud-free atmosphere to 4.9σ . When combined with other recent WFC3 studies, our observations suggest that clouds may be common in exoplanetary atmospheres.