Trimarcisia (, trimarkisia), i. e., "feat of three horsemen", was an ancient Celtic military cavalry tactic or organisation; it is attested in Pausanias' Description of Greece, where he describes the use of trimarcisia by the Gauls during their invasion of Greece in the third century BCE.<br> According to Pausanias: Pausanias' view was that the Gauls had adopted this method of fighting by copying the Persian Athanatoi elite force with the difference that while the Persians waited until after a battle to replace their casualties, the Gauls "kept reinforcing their full number during the height of the action".
According to Pausanias, marka was the Celtic name for a horse. This corresponds to the root *mark-os of words for "saddle horse" attested in Celtic and Germanic but not in other Indo-European languages, a root that is of uncertain etymology.