The letter highlights the growth of their relationship in the last 10 years. It also celebrates Anne becoming a mother.
Published as is, the chef-restaurateur's post reads: "A decade has gone by and I've seen you grow in so many different ways.
"From short hair to long hair and back, from late nights out to order in TV dinners, from petty young love fights, to laughing off problems we know we can work through together, from beer to kombucha, from insecurities to self confidence, from tight fitting dresses to comfortable pregnancy jeans and from your love of fish and chips, to your surviving love for fish and chips. The list goes on.
"Some things have changed, others have stayed the same, but you're always moving forward.
"I can't wait to see you go through the next couple of weeks, as you come into motherhood.
"I don't see it as a next step, or a new phase in life, more so as something you are adding to your growing list of accomplishments in life, because I know that there is still so much more to come.
"Happy birthday @annecurtissmith"
Anne has been in her hometown in Australia since she entered her third trimester, and is currently enjoying her maternity leave.
Erwan accompanied Anne to Melbourne, but he had to fly back to Manila some time in January.
Over the weekend, Erwan and Anne reunited in time for Valentine's Day.
For Valentine's Day, the couple attended a baby safety workshop, which taught them how to do baby CPR.
Erwan and Anne are expecting a baby girl.
The It's Showtime host is due to give birth in March.