"The story that appeared in this week's edition of In Touch magazine is not true," the couple's rep said in a statement to People.
The report came after Hamm, 44, credited Westfeldt for supporting him after he completed a 30-day alcohol addiction treatment program.
"The only gentleman Jennifer visited regularly in Connecticut was Jon, while he was in rehab. They continue to ask for the public's understanding and sensitivity during this challenging time," the spokesperson added.
"He needs [Jennifer],” a source told People of the stability of the relationship. "He has to have that in his life."
Westfeldt recently told GQ magazine that Hamm leaving the “darkness” of Don Draper on “Mad Men” was weighing heavily on the actor, saying, “The end was of course bittersweet and complicated for Jonny. But I know that when he wrapped the show in L.A., in the wee hours of July 3 last year, and then immediately got on a plane to New York to meet me and our dog, Cora, for dinner with a few close friends and to watch the fireworks, he felt a predominant sense of relief.”