"Absolutely brilliant, very nice thing to do ... well done to Ajax fans," wrote West Ham fan Dave Lewis on Twtter.
"Faith in humanity restored," wrote another user. "Shame it's the exception NOT the norm these days."
The Amsterdam club survived a spirited fight-back from the Ligue 1 side to progress to the final of the European competition 5-4 on aggregate.
Ajax will face Manchester United in the Europa League final, with Jose Mourinho's side needing to lift the trophy to make it into the Champions League next season.
United defeated Spanish side Celta Vigo 2-1 on aggregate, surviving a late semi-final second leg scare at Old Trafford, to make it to the final in Stockholm, Sweden.