Simmonds broke her own world record - which she set in the qualifiers - to take gold number two.
The win - which comes just two days after winning gold in the 400m freestyle - makes her the third Paralympian to taste double gold success alongside cyclist Sarah Storey and equestrian Natasha Baker.
The race was attended by Prime Minister David Cameron, who took time out from organising his Cabinet re-shuffle to cheer on our Paralympic heroes.
Mr Cameron kissed smiling Simmonds on both cheeks and shook her hand as he presented her with the gold medal.
On Tuesday, Simmonds - who has dwarfism, will be hoping to make it a stunning hat-trick of golds as she races in the 50m freestyle.