The fitted dress showed off her slim figure and was in a figure-hugging material.
The design is the TR STYLE 160 from boutique designer Pronovias.
On the designers website, the dress description reads: “An iconic mermaid-style crepe gown is transformed with an asymmetrical taffeta ruffle that elegantly frames back, neck and shoulders.
“A full train completes the look.”
The eye-catching design has a high-waisted style and was in a crepe material.
Amanda finished her look with a glamorous smokey eye and nude lipstick.
She wore her short blonde hair in loose curls that were pulled to one side.
The Britain’s Got Talent favourite kept her jewellery simple wearing only her diamond engagement ring and a thin gold bracelet.
Tonight, viewers can watch on for the second round of the semi-finals.
Amanda has sat on the judging panel of the show since 2006.
She is currently a judge with Alesha Dixon, David Walliams and Ashley Banjo.
Alesha also wore an eye-catching outfit for the semi-finals.
She turned heads in a strappy black bralette and stylish fuchsia co-ord.