The "bohemian" shower was held at Jessica's LA home, with Jessica and other bridesmaid sharing the day on Instagram.
"All the beauties celebrating @ashleesimpsonofficial / @missbananahammock!!! I love you Ash :)" Jessica captioned one shot which showed all the guests posing behind the swimming pool.
Around 20 guests were invited and were treated to floral wreath tutorials, reports People magazine.
A source also told the magazine: "The bridesmaids picked a throwback photo to decorate the cake", which bridesmaid Stephanie Pearson shared a snap of on social media site Instagram.
The bride-to-be will be marrying her musician fiancé Evan Ross, son of Diana Ross. The pair threw a star-studded engagement party earlier in 2014 to celebrate their upcoming wedding.
Ashlee and Evan, 29, announced their engagement in mid-January, when they posted identical snaps of themselves on Twitter. "My baby love and I are ENGAGED!!! Hallelujah Hawaii !!!!!" wrote Ashlee referencing Diana's famous song, her huge engagement ring proudly on display.
This will be the second time that Ashlee has tied the knot. She was previously married to Fall Out Boy rocker Pete Wentz, with who she has a five-year-old son Bronx.
Jessica also used the opportunity to share several pictures of herself in a black swimming costume.
The blonde beauty showed off her toned physique which she gained through exercise and following the Weight Watchers program.
"Oh yea girl...We are proud to be in our bathing suits!! @baylormcg #hardworkpaysoff #workoutpartnersforlife" Jessica shared with fans.