The Reds skipper offers his congratulations to the right-back.
The 23-year-old, whose existing deal was set to expire this summer, put pen to paper on a fresh three-year deal on Merseyside, potentially tying him to his boyhood club until 2019.
For Flanagan, it is light at the end of the tunnel, having endured an injury-ravaged last two years with Liverpool.
The highly-rated defender returned to action after 619 days out with a knee problem in the FA Cup replay at Exeter back in January, making four appearances since then.
Midfielder Henderson took to Instagram to congratulate his team-mate, writing: Well done mate! Thoroughly deserved!"
One of those appearances was against Manchester City in the Premier League at Anfield two weeks ago.
There, Flanagan reminded Liverpool fans of his passion with a hard tackle on ex-Red Raheem Sterling - a crunching challenge which Henderson applauded during the match.