Prenatal cannabis use is associated with health risks for mothers and their children. Prior research suggests that rates of prenatal cannabis use in Northern California increased during the COVID-19 pandemic, but it is unknown whether increases varied with the local cannabis retail and policy environment. To test whether pandemic-related increases in prenatal cannabis use were greater among pregnant individuals with greater retail availability of cannabis around their homes or among those living in jurisdictions that allowed storefront retailers. A cross-sectional, population-based time series study used data from pregnancies in the Kaiser Permanente Northern California health care system screened for cannabis use before (January 1, 2019, to March 31, 2020) and during (April 1 to December 31, 2020) the early COVID-19 pandemic. Proximity to the nearest retailer and number of retailers within a 15-minute drive from one’s home and local cannabis storefront retailer policy (banned vs permitted) were calculated. Interrupted time series models were fit using multiplicative and additive Poisson regression, adjusting for age and race and ethnicity. The COVID-19 pandemic. Prenatal cannabis use based on universal urine toxicology tests conducted during early pregnancy at entrance to prenatal care. The sample (n = 99 127 pregnancies) included 26.2% Asian or Pacific Islander, 6.8% Black, 27.6% Hispanic, 34.4% non-Hispanic White, and 4.9% other, unknown, or multiracial individuals, with a mean (SD) age of 30.8 (5.3) years. Prenatal cannabis use before (6.8%) and during (8.2%) the pandemic was associated with closer proximity to a retailer, greater retailer density, and residing in a jurisdiction that permitted vs banned retailers. There was a greater absolute increase in cannabis use from before to during the pandemic among those within a 10-minute drive (<10 minutes: adjusted rate difference [aRD], 0.93 cases/100 patients; 95% CI, 0.56-1.29 cases/100 patients; ≥10 minutes: aRD, 0.40 cases/100 patients; 95% CI, 0.12-0.68 cases/100 patients; interaction P = .02). Otherwise, relative and absolute rates increased similarly across categories of cannabis retailer proximity/density and local policy (interaction P > .05). Prenatal cannabis use was more common among individuals living in areas with greater retail availability of cannabis. Although relative rates increased similarly during the pandemic regardless of local cannabis retail and policy environment, there was a larger absolute increase associated with living closer to a storefront cannabis retailer. Continued monitoring of local cannabis policy, the retail environment, and prenatal cannabis use is needed.