The drowned robot was spotted by passers-by whose photos of the aftermath quickly went viral on social media.
The machine was apparently making its usual rounds outside the Washington Harbour office and retail complex in Georgetown when it tumbled down some steps and into the fountain, the Washington Post reported.
The robot began operating at the complex in recent days, it quoted an office worker as saying.
The Knightscope K5 is a rocket-ship shaped droid that stands around 4-feet tall. It is equipped with various hi-tech instruments - including face-recognition systems, high- definition video capture, infrared and ultrasonic sensors.
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"We were promised flying cars, instead we got suicidal robots," he said.
Others posted pictures of maintenance workers lifting the robot from the pool.
Knightscope, a Silicon Valley start-up, said in a statement that the incident was under investigation, pointing out that no one was hurt. The company called it "an isolated event" and promised to deliver a new robot this week for free.