The counties directly next to Napa County in California are:

  • Sonoma County to the west.
  • Solano County to the east.
  • Lake County to the northwest.
  • Yolo County to the northeast.

If you meant “next to the city of Napa” in a day‑to‑day sense (nearby areas people visit or live in), common nearby places include American Canyon (to the south, still in Napa County) and Sonoma in Sonoma County, which is a short drive over the hills.