Kate Cassidy is marking what would've been Liam Payne's birthday with a heartbreaking tribute ... sharing a carousel of photos from their time together.

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Kate posted the memories Friday, writing ... “your birthday will always be your day. loving you, remembering you, missing you always. happy birthday Liam ❤️”

She also shared a video of herself with a birthday cake ... lighting candles as she marked Liam’s birthday without him.

Kate’s carousel is filled with photos of her and Liam together ... looking back on moments from their relationship.

As TMZ previously reported ... Kate had been with Liam in Argentina before leaving the country just days before he died in October 2024 after falling from a hotel balcony in Buenos Aires.

Kate has continued honoring Liam ever since ... frequently sharing memories and emotional posts about their relationship.

And on his birthday, Kate is remembering the life they shared.