The new deal will keep the 18-year-old at West Ham until the summer of 2018.
Cullen earned his first call up to the Republic of Ireland U21 squad this week.
“I’m delighted to get the deal done and extend my time at West Ham,” said Cullen to West Ham’s official website.
“I’m over the moon to get another couple of years and I’m looking forward to the Club moving forward and hopefully progressing to the first team.
“The last couple of months have all gone quite quickly and I’ve had to take step back to take it all in. Obviously playing in the Europa League was a great experience, playing in front of a packed Upton Park was a special moment.
“Then going on at Anfield [for my Premier League debut] on Saturday was another moment that I’ll remember for the rest of my life.
“The whole of the last couple of months have been fantastic and I just hope that it can continue.
“I’ve just got to take it week-by-week, day-by-day as it comes. I’ve got to keep working hard, impress the manager playing for the Under-21s and hopefully I can get more first team opportunities.”