The 1865 Town of New Plymouth by-election was a by-election held in the Town of New Plymouth electorate during the 3rd New Zealand Parliament, on 19 May 1865. The by-election was caused by the resignation of the incumbent, Charles Brown, and was won unopposed by Henry Sewell. Whilst Sewell was not a local resident, he was a member of the government through his appointment to the Legislative Council, the upper house of Parliament. Sewell accepted the invitation to represent the electorate, as him becoming a member of the lower house was seen to strengthen the government.