It is the second year in a row that the now 22-year-old bags the prize.
"The bowl is not as heavy this year, or maybe I have become stronger," she said as she held the heavy glass bowl that comes with the top prize. "This is completely fantastic, wow. I do not know what to say. I want to thank all those who voted for me and who have been a big support for me in my venture. Without you I would never had managed to go on fighting with the sport, thanks a lot."
2015 was a big year for the swimmer. She won five medals in the World Championship in Kazan, two of them gold. She beat the world record twice in the 100 metre butterfly stroke, first while qualifying and then as she won the gold.
"I think what the Swedish people like is that it goes well for me at the championships and they can all the time see the results," she told Swedish Radio after stepping down from the stage.
So far, however, Sarah Sjöström has not had an olympic medal, but she is a Swedish favourite for the Olympic Games in Rio de Janeiro this summer.
"It is going really well. I was recently in Texas and competed and have had a great start to the year, I have never been this fast this early in the year in a competition, so now it is just to continue and try to swim even faster in the coming competitions," she told Swedish Radio's P1 on Tuesday.