Over the last decades, many parts of the world have witnessed a notable shift towards more punitive criminal justice policies. While public opinion is considered a key force shaping criminal justice policies, most individuals possess very limited knowledge of the everyday realities of life behind bars. This study explores the transformative impact of prison experience on public attitudes towards punitive justice policies. We take advantage of a unique randomized controlled trial offering regular citizens the opportunity to undergo up to two days of incarceration in a newly built prison, replicating the real-life journey of inmates. Our results reveal that subjects who received an opportunity to gain firsthand prison experience showed a significant change in punitive attitudes, becoming less supportive of harsh criminal justice policies and donating more money to organizations advocating for moderate justice policies. While participants generally overestimated prisoner wellbeing, this misperception was not significantly altered by the experiment. Our results underscore the crucial role of experiential learning in shaping support criminal justice reform.