Winger Hammill was tormentor-in-chief when the Tykes put Yorkshire rivals Leeds to the sword at Oakwell in midweek, rounding off the scoring in a 5-2 win.
The former Liverpool trainee this time denied Derby their first win since the opening day of the season by cancelling out Dean Leacock's first-half opener for the visitors with a measured finish early in the second period on Saturday.
Leacock's 14th-minute opener was his first-ever goal in nine years of professional football.
Barnsley failed to reach the same heights they scaled on Tuesday night against a hard-working Derby side determined to end a five-game winless run, but created more openings than their visitors, who started the day in the bottom three after successive league defeats.
Barnsley were unchanged after routing Leeds, while Derby pair Robbie Savage and Tomasz Cywka made way in midfield for Kris Commons and Ben Pringle.
Commons wasted little time in making an impact, lashing a 25-yard shot over the crossbar in the opening exchanges, but the visitors were on target soon after.
Barnsley goalkeeper Luke Steele punched clear Pringle's 14th-minute corner and Leacock kept his composure to head home a bouncing ball inside the penalty area into the bottom corner.
Barnsley defender Jason Shackell was then wayward with a long-range shot and Adam Hammill dragged his angled drive wide before Garry O'Connor forced Derby goalkeeper Stephen Bywater into a comfortable save.
Hammill kept Bywater on his toes with another shot, Jacob Butterfield tried his luck from 30 yards and O'Connor's goalbound header was cleared by Derby defender Shaun Barker as Barnsley chased an equaliser.
Derby held onto their lead until the break, but were pegged back just three minutes into the second period following some sloppy defending. O'Connor played in Hammill and the in-form winger curled a shot from 12 yards inside the far post.
The Tykes twice threatened to take the lead straight after the restart as Bywater kept out Butterfield's 20-yard shot at full stretch, while Barker again proved Derby's last line of defence when he blocked Jim O'Brien's piledriver.
Derby midfielder Paul Green went close with a 25-yard shot, Shefki Kuqi pulled his effort wide from inside the penalty area and Pringle was not far off target with another long-range drive as Derby enjoyed a spell of pressure midway through the second half.
Tykes manager Mark Robins replaced O'Brien and Andy Gray with Kieran Trippier and Liam Dickinson in the final quarter, but neither player was able to alter the course of the game and neither side was able to engineer further chances as the game ended in stalemate.