Bandaranaike International Airport is 7 kilometres south of Negombo — closer to the town than to Colombo. A tuk-tuk from arrivals to the beach strip costs around 500–600 rupees and takes 15 minutes. There are also metered taxis; agree the price before you get in unless using a ride-hailing app.
From Colombo, buses run frequently from the Bastian Mawatha bus terminal (Central Bus Stand) and take around an hour depending on traffic. The fare is cheap. Trains connect Colombo Fort to Negombo in about 45 minutes; the line terminates a short walk from the town centre.
If you're heading north from Negombo toward Kalpitiya or Puttalam, buses leave from the main bus stand near the market. There is no direct rail link north from Negombo.