He captioned the image: "Meet my little lad, Freddie."
Tomlinson then revealed his son is affectionately known as Freddie Tommo and thanked fans for all their "nice messages".
Jungwirth also shared a photo of her and Freddie in a hospital bed.
Tomlinson's Instagram post put to bed rumours he and Jungwirth had named their son Sydney Rain, which had been circulating since he confirmed the birth on 23 January.
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