كتب : Ahmad Saied
England's wonder kid was introduced with just 15 minutes remaining when the result was level to dish out two assists for Emmanuel Eboue and Julio Baptista and help Arsenal notch three precious points.
Dutchman Rafael van der Vaart had initially gave Hamburg a surprising lead before his compatriot Robin van Persie equalized after the break.
The result pushed Arsenal up to the summit of Group G with 10 points and left Hamburg rooted with no points.
In the same group, Porto beat CSKA Moscow 2-0 earlier on Tuesday. Both sides are still in the hunt with 10 and eight points respectively.
At the Emirates Stadium, Arsenal continued their bad habit of trialing in their new home when they conceded as early as the fourth minute.
Van der Vaart capitalized on a defensive blunder to unleash a superb shot and beat Arsenal keeper Jens Lehmann.
The Londoners laid siege to Hamburg's areas but failed to level the score line till seven minutes after re-start.
Van Persie was fed inside the area by Cesc Fabregas only to drill a low shot into the back of the net.
The 17-year-old Walcott, who replaced Freddie Ljungberg, was deployed on the right flank, launching several attacks that resulted in two goals.
He flicked the ball on to right-back Eboue inside the box and the Ivory Coast international blasted into the acute angle.
Walcott then swung an inch-perfect cross for the darting Baptista who headed his first Arsenal goal home two minutes from time.
In Moscow, Ricardo Quaresma broke the deadlock in the second minute before Lucho Gonzalez sealed the win one minute after the hour mark.