Miniaturization of polymeric robots leads to difficulties in actuation inside viscous media due to the increased surface drag on the diminutive robot bodies. Herein, agile underwater swimming of polymeric microrobots is presented with the investigation of correlation between the magnetic propulsion and viscous drag on the robot. The polymeric microrobots swim with pivoting and tumbling motions during underwater rotation by in‐plane rotation of two permanent magnets underneath the plane, which results in orbital revolution‐type locomotion with a maximum swimming velocity of 56 body lengths per second (BL s−1). The rotational ability and orbital velocity of the polymeric microrobots are determined by correlated variables, i.e., liquid viscosity and frequency of magnet rotation, as elucidated by experimental results and theoretical analysis. Based on the understanding of underwater orbital maneuvers, the polymeric microrobots achieve agile swimmability at a viscosity similar to that of normal whole blood and self‐correcting maneuverability in diverse vascular‐like environments, including a stenosed tube with a coarse granular hill and a rough‐walled artificial blood vessel. Agile underwater swimming can improve versatile aquatic performances of miniaturized robots in blood vessels with arteriosclerosis or blood clots.