The former couple shares a son, 10-year-old Axl Jack.
The "Fergalicious" singer took to Instagram to comment on the couple's baby announcement on Monday, offering a heartfelt congratulations on their growing family.
"I am truly happy for you guys🥰🥰 Axl can’t wait to be a big brother," she wrote.
The comment came one hour after Mari and Duhamel shared the news on Instagram, posting a sonogram image with the caption: "Baby Duhamel coming soon."
Mari also took to her Instagram Story to share a look at her growing baby bump while touring the Van Gogh: The Immersive Experience exhibition in Miami, Florida, with the actor.
Fergie and Josh tied the knot in 2009 and announced their split in 2017. The next year, Fergie filed for divorce from Josh, 50, two years after their initial announcement. Together they share a son, 10-year-old Axl Jack.
When ET spoke to Fergie in June 2018, she opened up about what it's like co-parenting with Josh.
"We split it up really balanced," she said. "We're all love and we love each other. We're all cool and just a family. I have a good co-parenting partner, so we work it out."
Josh echoed his ex's words, telling ET days later that they "get along great."
"We just love our kid and want the best for him," he said. "Fergie and I get along great. We have a lot of love and there's no reason to not do the best we can to raise our kid. He's no. 1."
"Fergie has always put health and parenting first in her life. She is finally ready to say [her marriage] is over for good and that's what led her to file for divorce," a source told ET in 2018. "Fergie finally had the strength to say her marriage to Josh was over once and for all. Those closest to her knew she would get to this point eventually. She feels good standing on her own and it was time to let go."
"Josh has been there for Fergie and they have managed to co-parent very well," the source added. "Josh loves Fergie and is happy she's moved forward. He's moved on and wants to remain a close friend to Fergie."
Mari and Duhamel celebrated their first anniversary on September 10.
"Happy First Anniversary sweet man," Mari wrote on Instagram this week. "Oh how I prayed for you.."