Experimental tests were conducted on seven 1/3-scale specimens to define the vulnerabilities of existing outrigger bents under in-plane and out-of-plane seismic loading and to develop appropriate retrofit measures that address the identified vulnerabilities. The specimens represented bents with short and long outrigger beams in the SR-99 Spokane Street Overcrossing in western Washington State. The as-built specimens failed at low ductility levels due to shear distress, low torsional strength of the beam, and reinforcement bond failures within the joint. Circular and D-shaped steel jackets were used to retrofit the regular and split as-built specimens, respectively. The retrofitted specimens developed plastic hinging in the column, with enhanced strength, energy and ductility capacities. Threshold principal tension stress values describing the expected condition of the joints were established and compared to values obtained by other researchers. Design and detailing guidelines for retrofitting outrigger bents were proposed. The guidelines include equations for the jacket thickness required to form a stable force transfer mechanism between the beam and the column reinforcement as well as to prevent joint failure.