The rare observation of transient Rh···Rh excimer formation in a single crystal is reported. The estimated excited-state lifetime at 100 K is 2 ns, which makes it the shortest-lived small-molecule species caught experimentally using the laser-pump/X-ray-probe time-resolved Laue method. Upon excitation with 390 nm laser light, the intermolecular Rh···Rh distance decreases from 3.379(4) to 3.19(1) Å, and the metal–metal contact gains more bonding character. On the basis of the experimental results and theoretical modeling, the structural changes determined with 100 ps time resolution reflect principally the S0 → S1 electronic transition.