The Americans lead the medal count with 19, six more than the Chinese.
Finishing first in the women's 100m breaststroke on Monday, King did not congratulate Efimova but hugged fellow American swimmer Katie Meili, who won bronze.
The Russian surged forward in the second half of the race and appeared to be catching her rival, but King finished powerfully in an Olympic record time of one minute 4.93 seconds, with Efimova 0.57 seconds behind.
King was captured shaking her finger at the Russian as she watched her qualify for the final, with Efimova holding up a "No 1" sign.
King made her move after 50 meters while Efimova made her move approximately 25 meters to go which was not enough to catch and beat King. Efimova, 24, who was loudly booed by spectators before the race, was only given permission to compete in the Games after appealing against a suspension.
She was joined by U.S. swimming legend Michael Phelps and other swimmers who have called for tougher action on the use of banned substances.
Phelps, who won his 22nd gold medal and 26th Olympic medal on Thursday night, has never tested positive for any kind of performance-enhancing drug in or out of competition.
Efimova served a 16-month suspension for doping from late 2013 to February 2015 and also failed a test for meldonium earlier this year (only for the result to be later overturned).
On Monday, she let her swimming do all the talking.
"Of course, I'm not for doping, and I've never used it on objective", Efimova said. IU and Bloomington should be very proud of her.
King and Efimova were pitted against each other in adjacent lanes. "There is a way to become the best and do it the right way", King said in a statement.
After winning her Olympic silver, Efimova hit back at her critics, saying that all athletes should be above politics. WADA initially banned the substance and then revoked its rules when there was no information regarding how long it stays in one's system. The Russian contingents weren't cheered at the Opening Ceremony, and this has continued in the different events as well. They have nothing to do in the sport. They had expected the body to be stricter when it came to doping, but since they are a higher body, the International Olympic Committee has no option but to follow their orders.