Highly pathogenic avian influenza H5N1 has infected US dairy cattle and spread throughout 19 different states since March 2024. Although within-farm transmission is thought to occur primarily through direct contact, local between-farm spread remains unknown, and is considered multifactorial and likely driven by routine business activities involving shared personnel, vehicles, and other conveyances. This investigation evaluated the presence of influenza A virus RNA in a convenience sample of milk transport trucks and other on-farm service vehicles in H5N1-affected areas in 3 states between May 2024 and April 2025. A total of 298 samples were collected from 52 vehicles, of which 38 were milk trucks and tankers from 2 dairy cooperatives. Influenza A virus RNA was detected in 1.3% (4/298) of samples: 2 from tires on a livestock hauler and a deadstock trailer in Michigan, 1 from a milk cabinet sample from a milk tanker in Michigan, and 1 from a tanker milk sample in California. Although a very small percentage of samples were positive, this investigation shows that H5N1 virus can be detected on vehicles servicing US dairy herds, highlighting a potential pathway for between-farm transmission.