After a stint in his home country with Dinamo București during which he earned three domestic honours, including one national title, Radu moved to Lazio on an initial loan in 2008.

Radu registered his senior debut for the Romania national team in 2006, and made thirteen appearances before retiring early in 2013, at age 26.

[4][5] On 27 April 2005, Radu made his debut for Dinamo București under coach Ioan Andone in a 3–0 league victory over FCM Bacău.

[4][5] In the 2006–07 Liga I season, helped by the fact that Dinamo had a defender emergency, he was established in the starting eleven by coach Mircea Rednic, making couple in the central defense alongside Cosmin Moți and eventually managing to win the Liga I trophy that season, also managing to pass the group stage of the 2006–07 UEFA Cup, reaching the sixteenths-finals where the team was eliminated with 3–1 on aggregate by Benfica.

[12] He was a revelation for Delio Rossi's team, as the young starlet added extra quality to the backline and started all the matches he featured in.

[16][17] Ștefan Radu played 13 games at international level for Romania, making his debut on 15 November 2006 under coach Victor Pițurcă who sent him on the field in the 89th minute in order to replace Răzvan Raț in a friendly which ended with a 1–0 victory against Spain.