When you give someone your "heartfelt thanks" it means that you are thanking them in a very deep, geniune, emotional way. This is a rather formal phrase that you can use when giving a speech, writing a formal letter, or making a public announcement.

There are a few ways to use "heartfelt thanks" in a sentence. Here's one – "my/our heartfelt thanks to ___":

Our heartfelt thanks to everyone who participated.

Here's another – "give ___ my/our heartfelt thanks":

I give you my heartfelt thanks for everthing that you've done for me over the years.

Other words that you can use with "hearfelt" include:

• my heartfelt apologies
• our heartfelt condolences