The 31-year-old Curaçao international midfielder, with 13 international caps, is the latest player to sign ahead of the 2018/19 season. "I've heard a lot about the club and the future plans" said the 2012/13 League Cup winner.
"I know Owain Jones and Curtis Obeng from my time at Swansea City and I've heard a lot about what a good coach Nicky is, so I'm excited to join and looking forward to playing in the United Kingdom again."
As well as a stint in Scotland with Hamilton Academical and England with Birmingham City, Crystal Palace and Brighton & Hove Albion, Augustien had a long spell in Wales with the Swans.
"I enjoyed my time at Swansea immensely. The style of play that Brendan Rodgers wanted to implement really suited me. He wanted to play possession football which is the way we're taught to play growing up in the Netherlands so it was perfect for me."
Agustien brings vast experience, having also plied his trade in Denmark, Philippines and his native Netherlands with the likes of AZ Alkmaar and Willem II added to his international appearances.
"I'm hoping to play my way back into the first team for Curaçao and help them qualify for the CONCACAF Nations League and Gold Cup." Agustien could face Boro' teammate Kairo Mitchell at the qualifying stage as Curaçao host Grenada in September.
Boro's busy summer continues as Agustien is now signing number seven since Nicky Eaden's arrival.
Updated 18:19 - 27 Jun 2018 by Adam Etheridge