Captioned, "Well little angel, another cumpleaños.. You inspire millions, and I'm one of them. Your smile keeps getting bigger and bigger with every birthday that comes by! I'm blessed to see it shine... 23, You totally look it, werk! I love you AND #YonceSoHardMoaf**kasWannaFineMe," the That 70's Show star clearly still has a fire lit for the "Cool for the Summer" singer.
The pair have been dating since 2010 - just after Lovato's 18th birthday.