The Democrat told The Boston Globe, which first reported the news, that he considered running in 2012 against the eight-term incumbent.
That cycle Tierney avoided a primary challenge, but he ended up facing a difficult fight in Republican Richard Tisei, whom he defeated by just over 1 percentage point.
Tierney, whose family was embroiled in scandal surrounding an offshore gambling ring, suffered from attacks highlighting his family troubles and significantly underperformed President Obama in 2012.
And Tisei, an openly gay moderate Republican, was one of the cycle's strongest GOP contenders. He's considering a rematch against Tierney, but hasn't yet indicated his plans publicly.
The Democratic Congressional Campaign Committee has named Tierney to their list of most vulnerable incumbents, preparing for what's expected to be another difficult battle in 2014.
And that battle becomes even more daunting with the entry of Moulton into the race. The challenger has hired a slate of seasoned Democratic operatives, according to the Globe, including Joe Trippi, former campaign manager for Howard Dean's 2004 presidential race; Max Glass, former manager of Rep. Tulsi Gabbard's (D-Hawaii) race; and Liberty Square Group, the firm led by Scott Ferson, who advised Rep. Stephen Lynch's (D) Senate campaign.
Rep. Tierney gets primary challenge
Seth Moulton, currently co-founder and president of a Massachusetts healthcare company, will announce his run for the 6th district on Tuesday, according to his website.