All lanes were shut on the M25 QE2 bridge from junction 31 A1306 (Lakeside / Purfleet) to junction 1A A206 (Dartford) last night (October 22).
Essex Police were called to the bridge just after 7pm to concerns for the welfare of a man.
Members of the public reported that a man had entered the water, the force said.
The bridge was closed for four hours as emergency services, including the coastguard, conducted an extensive search of the area.
Trapped traffic was being diverted through the right-hand Dartford tunnel during the closure.
A spokesperson for Essex Police said: “Our enquiries are continuing this morning.”
Although enquires are ongoing, the Dartford Crossing has not been impacted by major traffic delays this morning.
The bridge itself fully reopened at around 10pm last night.
