The 24-time Australian international makes it eight Matildas in the FA Women's Super League, joining Lydia Williams (Arsenal), Steph Catley (Arsenal), Caitlin Foord (Arsenal), Sam Kerr (Chelsea), Chloe Logarzo (Bristol City), Ella Mastrantonio (Bristol City), and Hayley Raso (Everton) in England.
19-year-old Galabadaarachchi is yet to represent Australia at senior level but has played in Under 20s.
“I am delighted to join West Ham United,” Arnold told West Ham's website.
“I feel like this move has been something I’ve been looking forward to for a long time and I don’t think I could have ended up at a more perfect team for me.
“Matt (Beard - coach), the staff and the girls have been so welcoming already and I can’t wait to get over to England to start training all together.
"I’ve heard some really good things about the Club and the incredible West Ham fans. I hope I can contribute to that and that West Ham will be somewhere I can call home for a long time to come.”
Arnold is a three-time W-League Goalkeeper of the Year and has played for Australia 23 times.
Other Matildas in Europe include Ellie Carpenter at Lyon, Alex Chidiac at Atletico Madrid, Mary Fowler at Montpellier, Kyah Simon and Amy Harrison at PSV.
Until recently, Lisa de Vanna was at Fiorentina and Emily Gielnik was with Bayern Munich.
Emma Checker also plays in France for FC Fleury 91, Elise Kellond-Knight is signed up with Kristianstads in Sweeden and Katrina Gorry, Clare Polkinghorne, Tegan Micah, Karly Roestbakken, Tameka Yallop and Nikola Orgill are all signed up to play in Norway.
