A Learjet 25D, registration YV1738, was substantially damaged during a runway excursion during landing at San Juan-Fernando Luis Ribas Dominicci Airport, Puerto Rico. The two pilots, and one passenger were not injured. Day, instrument meteorological conditions prevailed at the time, and an instrument flight rules flight plan was filed. The flight originated from Punta Cana, Dominican Republic, about 17:30. The pilot reported that the runway appeared to be wet as they were cleared to land. There was light rain on the windshield, and the rain intensified as they approached the runway. After a smooth landing, as the airplane decelerated through 90 to 80 knots, it began to hydroplane, and departed the runway surface to the left, into the grass. As the airplane crossed the intersecting taxiway, the left main landing gear collapsed, the airplane then completed a 180-degree turn, and came to rest in the grass. The crew and passenger egressed the airplane and were met by first responders. The left main landing gear actuator support beam was structurally damaged. There was also damage to the left wing tip tank. Rudder control continuity was confirmed. An initial examination of the main landing gear tires revealed that three of the four tires were worn to the point where no visible tread remained. The weather at San Juan, at 18:20 included, wind from 350 degrees at 8 knots, 2 statute miles visibility in thunderstorms, overcast ceiling at 2,000 feet above ground level, and altimeter 29.94 inches of mercury. PROBABLE CAUSE: “The pilot’s improper decision to land the airplane on a wet runway, in heavy rain, with tires worn beyond safe limits, which resulted in a hydroplaning condition and subsequent loss of directional control.”