Last year, the Finn signed a one-year contract with an extension option for 2020, which Mercedes have exercised.
The 30-year-old has been with the German team since 2017, and has made a strong start to the 2019 season with four pole positions and two race wins, in Australia and Azerbaijan.
Bottas lies second in the championship, 62 points behind team-mate and defending champion LeWIS Hamilton.
Mercedes team principal Toto Wolff said: “For 2019, we set Valtteri the challenge of coming back even stronger than we saw him in the first part of 2018 – and he has achieved that, with some really impressive performances in the early races this year.”
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Bottas, who is level with Hamilton on four pole positions going into the Belgian Grand Prix this weekend, said: “I am very happy and proud to be part of the team for a fourth season and WISh to thank every team member and the board of Mercedes for their trust and belief in me.
“My performances have been getting better and better each year, and this is a great way to kick-start the second half of 2019.”