The 46-year-old will work alongside head coach Marianne Delaney-Hoshek and a new-look squad.
Seymour was part of the coaching team from 2010-2014 and said it was a "privilege to be reappointed''.
"I’ve always had passion for netball here in the Mainland and am really looking forward to working with the team next year."
Delaney-Hoshek said she was excited about Seymour's return.
"She is a coach who lives out her values and will help instil hard work within the group," she said.
"We worked together for the national secondary schools as a successful team and I am looking forward to her re-joining the Tactix family."
Seymour said joining Delaney-Hoshek was a "big factor" in her applying for the position.
"I have coached the NZ Secondary schools team with Marianne and I think we have a great coaching relationship."
Netball Mainland chief executive Brigit Hearn said she was delighted to have a "playing and coaching icons" back in the fold.
"Excitement levels are high to start pre-season training for the 2018 season."
Seymour said it would be an opportunity to "re-connect" with some familiar faces and work with the new players on the team roster.
She recently spent time as assistant coach of the victorious New Zealand U21 World Youth Cup team.
She was assistant coach with the New Zealand FAST5 Ferns in 2013 and 2014 when the side won the Netball World Series title.
The Tactix will begin pre-season training in late January with the 2018 ANZ Premiership set to start in May.