JupiterResearch, which is headquartered in New York, has 82 employees and posted revenues of about $14m in 2007.
Forrester, with 2007 revenues of $212m, now has more than 1,000 employees.
Jupiter joins Forrester's Marketing and Strategy Client Group, which contributed $46.4m to Forrester's total revenue in 2007.
George Colony, chairman and chief executive of Forrester, said: "Uniting JupiterResearch and Forrester brings together the two leading research brands used by marketing and strategy executives.
"JupiterResearch enhances our existing role-based strategy and offerings to bring unprecedented value to Forrester's Marketing & Strategy clients. We welcome JupiterResearch employees and clients to Forrester."
Both Forrester and JupiterResearch provide technology-focused market research and consulting services.
Separately, Forrester reported its pre-tax profit for the second-quarter ended June 30 2008 rose to $8.6m, up from $4.5m in the same period a year ago. Its revenues also grew 15% to $63.5m.