The 23-year-old wowed in a green chiffon dress by Belgian designer Natan, with the floaty number featuring a caped overlay and a tiered skirt.
Belgian royal news
She teamed the frock with gold skyscraper heels and matching accessories.
Elisabeth, who is the heir to the Belgian throne, joined her parents, King Philippe, 65, Queen Mathilde, 52, and her younger siblings, Prince Gabriel, 21, Prince Emmanuel, 19, and Princess Eleonore, 17, at the Te Deum mass at Saint Michael and Saint Gudula Cathedral in Brussels.
The princess recently returned home from completing her first year of her two-year master's degree at Harvard University in the US.
In May, President Trump's administration put a ban on international students attending classes at Harvard on a visa. The prestigious Massachusetts university pushed back with a lawsuit, meaning the rule was blocked temporarily.
Elisabeth has since carried out numerous public engagements in Belgium and recently completed an internship with Bruegel, an independent research centre in Brussels, which specialises in research of (European) economic policy.
Elisabeth accessorised her look with a go-to royal accessory – a gold raffia headband. She also borrowed her mother's statement earrings as well as a CH Carolina Herrera clutch bag, according to Belgian news site, HLN.
The Belgian royal family posed for snaps on the steps of the cathedral.
Noticeable absentees were King Philippe's parents, King Albert, 91, and Queen Paola, 87.
Since his abdication in 2013, Albert and his wife have largely retired from public life, with the former monarch having recently undergone surgery for skin cancer.
National Day takes place on 21 July in Belgium because that's when Leopold I took the oath as the first King of the Belgians in 1831.
King Philippe also ascended the throne on 21 July 2013 so the date is extra meaningful for him.
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Prince Albert, Princess Charlene, and their children, Princess Gabriella and Prince Jacques, attended the inauguration of the Christmas Lights on the Place du Casino