The singer and the US actor have been dating since last summer, and Allen has been pictured wearing a diamond engagement ring on several occasions this year.
Allen has since confirmed the reports by sharing a series of snaps from the special day on her Instagram page – check them out below.
The singer reportedly used her previous married name – Lily Cooper – for the registration. She separated from husband Sam Cooper in 2015 after four years of marriage, before they eventually divorced in 2018.
The pair have largely kept their relationship under wraps, only sharing a selection of photos on social media and appearing in public together on several occasions.
When Allen was quizzed about the large ring on her finger in a photo earlier this year, she replied: “First rule of engagement club…”
Meanwhile, it is expected that Harbour will return as police chief Hopper in the fourth season of Stranger Things, despite facing an uncertain fate at the end of the last season.
In a clip released earlier this year, it was revealed that his character is now interred in a Russian labour camp.