The couple made their relationship red-carpet official at a Vanity Fair party in February 2022.
The Layer Cake star debuted her growing baby bump during a trip to the beach in Ibiza on Monday.
Miller, who is also known for appearing in 21 Bridges, is already mother to daughter Marlowe, 10, who she shares with her ex Tom Sturridge.
British Vogue was among those to confirm and congratulate Miller on her happy news.
A Tweet from the publication read: “Congratulations are also in order for former #BritishVogue cover star #SiennaMiller, who is expecting a baby with her boyfriend Oli Green. It will be a second child for the actor, who has a 10-year-old daughter, Marlowe.”
Congratulations are also in order for former #BritishVogue cover star #SiennaMiller, who is expecting a baby with her boyfriend Oli Green. It will be a second child for the actor, who has a 10-year-old daughter, Marlowe. pic.twitter.com/KbRWfsnm8o
— British Vogue (@BritishVogue) August 22, 2023
The star has previously spoken about freezing her eggs after turning 40.
She said: “Biology is incredibly cruel on women in that decade [30s] – that’s the headline, or it certainly was for me.
“Then I got to 40 and I froze some eggs. Having been really focused on the need to have another baby, I’m just like, if it happens, it happens. That kind of existential threat has dissipated.”
Miller is one of the most high-profile victims of the tabloid phone hacking scandal.
She said last year that the experience led her to “an absolute breakdown on every single level”.
Speaking about being targeted by the now-defunct News of the World, Miller told British Vogue exactly how the experience affected her at the time: “A couple of years of absolutely chaotic behaviour.
Sienna Miller Is Pregnant! Actress Expecting Second Baby as She Shows Off Bump While on Vacation (Exclusive) https://t.co/5WlX28hvOh
— People (@people) August 21, 2023
“I did not know which way was up or down.
“I was, I suppose, in the midst of an absolute breakdown on every single level…
“Couldn’t say what had actually happened. My way of dealing with that was to slightly lose it.
“And I did, I was running around the Vanity Fair party with no shoes on and getting really pissed.”
Miller was one of a number of celebrity victims who had the voicemails of their personal phones hacked by reporters from News of the World and other publications owned by Rupert Murdoch.
Miller shared her thoughts on the period, saying: “There are some things I regret, because I wish I’d been more protected.
“But life was so out of control. It’s a miracle that I actually retained a career and a life.”