Thankmar (or Tammo) (c. 908 – 28 July 938) is the eldest (and only) son of Henry I of Germany by his first wife, Hatheburg (or Liutgard).

His mother had been previously married and widowed, after which she entered a convent.

Because she left the convent to marry Henry, her second marriage is considered invalid and the couple split.

Thankmar's legitimacy is, therefore, in question.