Halliday has agreed a two-year deal with Hearts, while Beattie has left the Edinburgh club to sign a permanent deal with Aberdeen.
Attacking midfielder Halliday has earned 13 Northern Ireland caps since making her debut against Albania in 2023.
The 20-year-old has spent her entire career at Linfield, making 85 appearances, scoring 53 goals and assisting on 26 occasions.
“We’re really pleased to bring Keri in,” said Hearts manager Eva Olid.
“She’s a direct, positive player who always looks to make things happen in the final third. She’s confident on the ball, not afraid to take on defenders, and has a real instinct for finding space in and around the box.
“Although she’s still young, she’s already shown she can handle the demands of senior football, and we believe our environment will really help her develop further.
“Keri fits the kind of profile we’re looking to invest in — players with ability, work rate, and the right attitude to grow with us.”
Northern Ireland’s Young Player of the Year in 2024 joins fellow Northern Ireland internationals Joely Andrews and Lauren Wade at the Scottish Women’s Premier League 1 club.
Hearts’ head of recruitment Sean Burt added: “Keri is a player we’ve been looking at for a while and are really pleased to bring her to the club. She’s got great energy, a real competitive edge, and carries herself well both on and off the pitch.
“She’s widely regarded as one of the most exciting young prospects in the UK, and it’s clear she has the right mentality to match that potential. What stood out in our conversations was her attitude to learn, push herself, and be part of a team environment.
“We’re confident she’ll thrive here and are excited to support her journey at Hearts.”
Beattie has returned to Aberdeen on a permanent transfer following her departure from the Jambos.
The 22-year-old former Glentoran ace, who has signed a two-year deal with the club, has 11 caps for her country and scored four goals at senior international level.
She joined the Dons on loan in January this year and scored nine goals in 18 games to ensure Aberdeen remained in the SWPL1, as well as also reaching a first ever Scottish Cup Semi-Final.
Beattie made history by becoming the first Aberdeen player to receive a senior international call up.
“I’m really happy to be here. Finally, the day has come, and I am really excited to get started again,” said Beattie.
“I had made my decision in April that this is where I wanted to come. It is the first time in my career that I have been away from home and where I’ve had happiness both on and off the pitch which is when I thrive.
“Everyone here has such a tight bond, it’s a special feeling so to be part of that is amazing.
“The club is moving in the right direction and that is something I really wanted to be a part of.
“I feel like I love the club. There is a special connection, and I’m so excited for the new season.”