Scroll down for the full list of contestants.
One relative in particular was especially excited to see Molly selected — her grandmother. Addressing her thousands of fans, the singer-songwriter from Leicestershire posted a sweet message on Twitter.
"My Nan's called, she's over the moon. Forget national press, radio and TV— they made an announcement at...THE BINGO! Haha," wrote Molly.
The pop singer will perform her own song Children of the Universe at the 59th Eurovision Song Contest in May, held in Copenhagen. The semi finals will be split over 6 May and 8 May, with the grand finale taking place on 10 May.
"To represent the United Kingdom in such a huge competition, not only as a singer and performer but as a songwriter is an unbelievable honour," said Molly. "I hope I can do us proud."
The pretty star, who has supported Jake Bugg, Tinie Tempah, Labrinth and Chase and Status at previous gigs, has already had a taste of the limelight. She scored a place in the UK top ten when she sang on Sash's Raindrops (Encore Une Fois) in 2008.
Molly also won the urban/pop category at the Live and Unsigned contest at the O2 Arena in 2012, and sang on Basshunter's 2009 album Bass Generation.
The UK last won Eurovision in 1997, when Katrina and the Waves scored top points with Love Shine A Light.
While the island nation and other countries such as Spain, France and Malta have selected their singing hopefuls, others are still in the process of deciding. A total of 37 countries will participate.
This year's contestants so far:
Albania: Hersiana Matmuja with Zemërimi i një nate
Armenia: Aram Mp3 with TBC
Austria: Conchita Wurst with TBC
Azerbaijan: Dilara with TBC
Belarus: Teo with Cheesecake
Belgium: TBC on 16 March
Denmark: TBC on 8 March
Estonia: Tanja with Amazing
Finland: Softengine with Something Better
France: Twin Twin with Moustache
FYR Macedonia: Tijana Dapcevic with To the Sky or Tami Kaj Sto Pripagjam
Georgia: The Shin with TBC
Germany: TBC on 13 March
Greece: TBC on 11 March
Hungary: András Kállay-Saunders with Running
Iceland: Pollapönk with Enga fordóma
Ireland: Can-linn feat Kasey Smith with Heartbeat
Israel: Mei Finegold with TBC
Italy: Emma Marrone with La Mia Citta
Latvia: Aarzemnieki with Cake to Bake
Lithuania: Vilija Mataciunaite with Attention
Malta: Firelight with Coming Home
Moldova: TBC on 15 March
Montenegro: Sergej Cetkovic with TBC
Netherlands: The Common Linnets with TBC
Norway: TBC on 15 March
Poland: Donatan and Cleo with Slavic Girls
Portugal: TBC on 15 March
Romania: Paula Seling & Ovi with Miracle
San Marino: Valentina Monetta with TBC on 14 March
Slovenia: TBC on 8 March
Spain: Ruth Lorenzo with Dancing in the Rain
Sweden: TBC on 8 March
Switzerland: Sebalter with Hunter of Stars
Ukraine: Maria Yaremchuk with Tick-Tock
United Kingdom: Molly Smitten-Downes with Children of the Universe