The world record holder had qualified fastest for the final by more than four seconds as she looked to add to her S8 400m Freestyle silver from earlier in the Games.
Russia’s Olesya Vladykina led the race out but Frederiksen began to assert her dominance at the half-way stage, turning first in an S8 50m Backstroke world record 37.05 seconds.
And the Briton powered clear down the back 50m, winning by a body length as she touched in 1:17 seconds from USA’s Jessica Long (1:18.67) and Vladykina (1:20.20).
“That race was absolutely everything to me,” said the Merseysider. “Yesterday I was really nervous coming in to it, I was nervous this morning and I was nervous again tonight.
“But I was determined that I was going to fight to win it and that’s what I did.
“I knew I had to go out strong and I had a funny feeling the girls were catching me down the last 25m and last 15m because the crowd were getting louder and louder.
“So I just had to keep pushing and get to that wall.”