ValueClick's revised seven-strong board of directors will include three members from Be Free, including chairman and chief executive Gordon Hoffstein.
Be Free employs 12 staff in Europe, mostly based in the UK.
ValueClick has 46 staff in Europe, including 25 in the UK.
Carl White, European managing director at ValueClick, said: "Both European operations are fairly lean, so the focus is not likely to be on redundancies, but on how we can offer a wider range of products to marketers."
The new company will be known as ValueClick, and White will become European managing director of the combined operation. Be Free's UK clients include British Airways, lastminute.com and bol.com.
White said that Europe now accounts for almost one-quarter of ValueClick's business. He added that the UK business is profitable and that revenue has shown incremental growth every month. In the three months to 31 March, ValueClick posted global sales of $12.4m (£8.5m), and a pre-tax loss of $1.3m (£890,000).