Beattie, 39, worked with Monk at Swansea City and Leeds United, and joins David Adams, who left Everton in March, on Boro's coaching staff.
The ex-Southampton player retired in 2013 and later had a spell as Accrington Stanley manager.
Former Southend goalkeeper Darryl Flahavan, who also worked at Leeds, has joined as Boro goalkeeping coach.
Monk has also appointed Sean Rush as head of physical performance and Ryan Needs as head of performance analysis.
Middlesbrough, who were relegated from the Premier League last season, will begin their Championship campaign against Wolverhampton Wanderers on 5 August.
Middlesbrough manager Garry Monk has brought in former England striker James Beattie as first-team coach.