Munster chief Declan Kidney told the province's website, www.munsterrugby.ie: "We are delighted to sign someone of his calibre.
A lot of credit must go to Munster Rugby CEO Garrett Fitzgerald and outgoing manager Jerry Holland for acquiring Doug's services in such a complex market.
"There's a lot of rugby to be played in the next few months, however we are looking forward to welcoming Doug to the Munster fold on January 1st.
Howlett added: "This was a very big decision to make. I have had an amazing rugby career in New Zealand and I am grateful to every person who has worked with me and contributed to that.
"I consider next year at Munster as a fantastic opportunity to play for another great club and experience a different lifestyle with my family."