A jury found Seth Mazzaglia, 31, guilty of two counts of first-degree murder and two counts of conspiracy involving tampering with witnesses and physical evidence. He was led from the courtroom in handcuffs immediately after the verdict.
No date was set for sentencing. Mazzaglia faces life with no parole.
Marriott, 19, of Westboro, Mass., disappeared in October 2012. Prosecutors said Mazzaglia killed her and sexually assaulted her because she refused to continue a game of strip poker.
The body has never been found.
The three-week trial featured lurid testimony. Kathryn McDonough, Mazzaglia's former girlfriend, described helping him get rid of Marriott's body by putting it in a suitcase and dumping it in the estuary of the Piscataqua River at Pierce Island in Portsmouth.
McDonough also said she used Marriott's phone to send a text message in another effort to help Mazzaglia. The two young women got to know each other while working at Target.
Marriott's relatives, including her father and an uncle, hugged after they heard the verdict.