There is growing interest in the use of unconditional cash transfers as a means to alleviate poverty, yet little is known about the effects of such transfers in the U.S. This paper reports on the results of a randomized controlled study of a one-time $1,000 unconditional cash transfer in May 2020 to families with low incomes in 12 U.S. states. The families were receiving, or had recently received, Supplemental Nutrition Assistance Program benefits. We examine the impact of the cash transfer on five pre-registered outcomes (material hardship, mental health, parenting, child behavior, partner relationships) and several secondary outcomes (hardship avoidance, consumption, employment, benefit use). We find no statistically significant effects (powered to detect effects of 0.09 standard deviations) of the cash transfer on any outcomes for the full sample. In pre-specified exploratory analyses, we find significant reductions in material hardship (-0.17 standard deviations) among families with less than $500 of earnings in the previous month, roughly the bottom 50 percent of monthly earnings for the study sample.