Dyer, who won the ITV2 dating show in 2018 alongside then-partner Jack Fincham, has been dating Kimmence since last year.
Sharing a photo of her and Kimmence holding a scan, the 23-year-old wrote: "Little bubba can’t believe you are going to be ours.
"Me and Sammy are so excited to start this next chapter in our lives, feel so lucky and grateful."
A second photo showed a close up of the sonogram.
Dyer appeared to hint that the baby is due next year, adding "2021" to her caption.
The former reality star, who is the daughter of actor Danny Dyer and his wife Joanne Mas, previously dated Kimmence before her stint in the Love Island villa.
The couple then rekindled their romance following Dyer's split from her co-star Fincham in April last year.
Dyer's fellow Love Island contestants were quick to share their congratulations on social media, with friend Georgia Steel writing: "Awh baby I love you. Can't wait for this next chapter."
Megan Barton Hanson wrote: "Oh my goddess! Congrats babe your [sic] going to be the best Mumma."
Zara McDermott added "I'm so happy for you Dan," while Hayley Hughes commented with: "Ahh amazing news!! So happy for you, congratulations."
Dyer's ex Fincham welcomed a baby daughter named Blossom in January.