This article presents and analyzes all of the available evidence on Thracian participation in the Second Macedonian War. It is demonstrated that contingents (lightly armed infantrymen) from Thrace fought (as auxiliaries, and probably as mercenaries) on the side of the Macedonians, remaining reliable allies throughout the war. During the war, Roman diplomacy succeeded in organizing attacks on Macedonia by land from the west, north, and south, but not from the east; this observation proves Philip V’s strong positions in Thrace. Due to incomplete data, it is not possible to determine the exact number of Thracians in the Macedonian army or whether the number varied during the war. The article ends with a suggestion that the good awareness of the Romans about the geopolitical situation in the Balkans, demonstrated by T. Quinctius Flamininus, implies their acquisition (probably for the first time) of information about Thrace and the Thracians.