Healthy default beverage laws are relatively new interventions designed to improve the healthfulness of children's meals in restaurants. In this study, researchers assessed adherence to healthy default beverage laws among children's meals ordered online in Los Angeles (California statewide law effective from September 2018), Baltimore (effective from April 2018), and New York City (effective from April 2019) compared with that in Boston, where no law existed. Between November and December 2020, researchers ordered children's meals from online ordering platforms (e.g., GrubHub, Uber Eats) from the top-grossing restaurant chains in each location (n=337 meal orders from 106 restaurants), coded these meals using four successively stricter definitions of adherence to the default beverage laws in each respective jurisdiction, and then applied each law to data collected in Boston to simulate different policy scenarios in a city with no such law. The team analyzed these data in late 2021. Differences in adherence existed across jurisdictions, with 15% adherent in Los Angeles, 30% in Baltimore, and 43% in New York City, compared with 7%-30% in Boston, using the most lenient definition of adherence. Fewer than 3% of all meals adhered to laws when applying the strictest definition of adherence. Overall adherence was low and variable across jurisdictions. Adherence may be lower in jurisdictions with fewer allowable default beverages, although more research is needed to assess this potential causal relation. In addition to increased resources and support for restaurants, additional policy design considerations may be necessary to increase adherence to healthy default beverage laws.