The former Sale, Northampton and Bath hooker has made more than 100 appearances for the Pirates over the past three seasons.
However Ward will now return to the Premiership to compete with Joe Gray, Rob Buchanan and Chris Brooker for the No.2 jersey.
The 26-year-old said: "I'm very excited about joining Harlequins and working with Conor O'Shea, John Kingston and the coaching team and players.
"The team plays an attractive style of rugby that suits me and that combined with the club's current position at the top of the Aviva Premiership, makes this move an exciting opportunity for me."
Director of rugby O'Shea believes Ward will fit in well at the Stoop.
"With the recent retirement of Matt Cairns and the long-term injury to Chris Brooker, we needed a talented player to complement our existing front row," he said.
"Dave fits the profile of the type of player we want at this club and his style of play fits ours.
"He has the drive and ambition to play at the very highest level and we look forward to seeing him work with Joe Gray, Rob Buchanan and Chris Brooker next season."
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