Australian actress Rebel Wilson has revealed her exciting new role — as a mother — in a surprise announcement on Instagram.

Wilson welcomed her “beautiful miracle”, baby girl Royce Lillian, who was born in the past week via surrogate.

“I can’t even describe the love I have for her,” posted 42-year-old Wilson, who is in a relationship with girlfriend Ramona Agruma.

Wilson revealed to her 11.2 million followers that becoming a first-time mum had been “years in the making” and involved a number of people.

“I am forever grateful to everyone who has been involved, (you know who you are),” she wrote.

The Pitch Perfect star thanked her “gorgeous surrogate” who she praised for carrying the child to term “with such grace and care”.

“Thank you for helping me start my own family, it’s an amazing gift. The BEST gift!!

“I am ready to give little Roycie all the love imaginable. I am learning quickly…much respect to all the Mums out there!

“Proud to be in your club.”

The exciting news comes in a year of ups and downs for Wilson.

In June she was forced to reveal she was in a same-sex relationship with Agruma, who she referred to as her “Disney Princess”, after a journalist threatened to out her.

She recently told The Australian newspaper the reporter had engaged in “grubby behaviour”.

“Basically, with the situation where a journalist is threatening to out you, you’ve got to hurry, and some people we didn’t get a chance to tell before it came out publicly. And that’s not ideal,” she said.

Wilson was forced to reveal her fashion designer girlfriend’s identity on Instragram before Sydney Morning Herald columnist Andrew Hornery broke the story.

She posted: “I thought I was searching for a Disney prince…but maybe what I really needed all this time was a Disney princess.”

In September, Wilson spoke candidly about being offered more serious acting roles since she lost a considerable amount of weight.

Speaking to the PA news agency, she said: “I kind of feel like transitioning into different kinds of roles and into more dramatic roles and it’s really awesome. Of course I’ll still be doing comedies as well.

“But I think it will kind of change the roles that people see me being cast in.”

-with AAP