The 1878 Grey Valley by-election was a by-election held on 22 May 1878 during the 6th New Zealand Parliament in the West Coast electorate of .
The by-election was caused by the resignation of the incumbent MP Martin Kennedy on 13 April 1878.
The by-election was won by Richard Reeves.
He was opposed by William Henry Harrison (who had been elected for the electorate in the ) and John Barrowman; Heber Newton withdrew but got 25 votes.
The following table gives the election result: