Dag Thomas Hallberg (born 1 August 1940) is a former tennis player and administrator from Sweden.

[1] Hallberg represented Sweden in the 1961 Europe Zone final against Italy.

In the doubles rubber he partnered with Jan-Erik Lundqvist and lost in four sets to Nicola Pietrangeli and Orlando Sirola.

In the final singles rubber, he played against Fausto Gardini and lost in straight sets.

[3] Hallberg's one tournament victory was at the 1962 Kassel Open in Germany when he beat the Hungarian player, Istvan Sikorski, 6–2, 6–1 in the final.