The duet was released in June last year as a remix of Fender’s solo album track.
It is a two-way conversation about the end of a relationship, with Fender noting previously that the song is “all the better” for Dean’s input.
Fender and Dean debuted the collaboration in early June during one of Fender’s London Stadium shows, with the video of their performance going viral on social media.
Rein Me In is Fender’s first number one, while the pop-rock ballad is Dean’s second chart-topper after the success of her single Man I Need.
The pair also broke more records on Friday, achieving the longest consecutive run in the UK top 40 and the most weeks in the UK top 10 for any non-Christmas single.
There is one record left for the pair to break – the record for the longest running number one by any artist, which is currently held by American Frankie Laine, whose song I Believe spent 18 weeks at the top of the charts in 1953.