The East Coast Bays by-election of 1980 was a by-election during the 39th New Zealand Parliament in the East Coast Bays electorate. It resulted in an upset for the National Party, as their candidate and future leader Don Brash was unexpectedly beaten by Gary Knapp of the Social Credit Party.

The 39th New Zealand Parliament was a term of the Parliament of New Zealand which began with the general election held on 25 November 1978, and finished with the general election held on 28 November 1981. The dates of the Muldoon Ministry were from 13 December 1978 to 11 December 1981.