White Young Green said its Greek team was focusing on transport and environmental work, including airports, highways, hotels and ports projects.
International director Mario Donnetti said work needed for the Olympics included the construction of "overlay" projects, whereby existing sports venues are upgraded for the games.
The new Athens office will be run by Yanni Papapanagiotou.
Donnetti, the former international director at engineer Mott Macdonald, said the £46m-turnover group was also considering a move into Eastern Europe, and mentioned Poland as a possible target.
"We are trying to concentrate on particular countries and markets, rather than adopting a scattergun approach to growing internationally."
He added that the firm would look at foreign acquisitions once it had established operations and association in particular markets.