The 26-year-old from Wales lost to Japan's Mama Umeki having reached the final in the -78kg category on Sunday.
Powell's British team-mate Amy Livesey took -63kg bronze on the opening day on Saturday.
Powell, her team-mates and rivals are building up to the 2017 World Judo Championships in Hungary, in August.
Cancun in Mexico will host the next Grand Slam event, starting on 16 June.
Powell, who won bronze at the European Championships in April, said: "I'm really pleased to pick up another Grand Slam medal.
"It was great to see strategies I've been working on in training coming through into competition.
"I'm now looking forward to the final preparations for the World Championships."
Commonwealth Games judo champion and Olympian Natalie Powell has won silver for Great Britain at the Ekaterinberg Grand Slam in Russia.