In preparation for July's Pan Am Games in Toronto, the Victoria, B.C. native won the freestyle in 7:52.51 to post the sixth-best time of 2015.
The 26-year-old beat out Great Britain's Stephen Milne by over three seconds and took more than five seconds off the previous pool record set in 2011.
"The 800 is a race I'm looking forward to at worlds this summer," Cochrane said, referring to the FINA World Championships in Kazan, Russia beginning in July.
"And I'm happy to be able to break the Mare Nostrum record in the process."
Teammate Hilary Caldwell, also from Victoria, was the second podium of the day for Canada. The 24-year-old claimed silver in the women's 200-metre backstroke.
With a time of 2:09.36, Caldwell finished close behind Hungary's Katinka Hosszu, who touched the wall at 2:08.41.
"I am pretty happy with that swim," Caldwell said.
"I've done a lot of work on keeping my stroke rates consistent and that showed. It was a bit faster than I was at [Team Canada] Trials [in April] where I was tapered. So that's always a good sign for faster racing this summer."
Canada's Alec Page was 14th in the men's 100-metre butterfly, while 16-year-old Mary-Sophie Harvey finished 15th in the women's 200-metre individual medley.
The final leg of the Mare Nostrum Series takes place in Monaco on Saturday and Sunday.