The Shaw Lane side showed strength throughout the team with a spread of good bowlers restricting Castleford to an achievable target which their batsmen then overhauled with ease.
Batting first, the West Yorkshire side were all out for 167 in the f
inal over.
Mark Lawson took 3-50, Greig Batty 2-25 and Joe Ellis 2-35 and Barnsley wasted no time in chasing the simple target.
They reached 169-3 to win the match in the 44th over, with Simon Widdup top scoring on 52, while Alex Morris added 42 and youngster Steven Nuttall hit 34 not out.
Third-placed Doncaster Town dropped two points after failing to bowl out rivals Driffield and settling for a six-point win on Saturday.
Doncaster had looked set for a comfortable win having totalled 293-5 in their 55 overs but stubborn Driffield batsmen avoided losing their last wickets and ended on 182-8 to restrict their rivals to just a six point win.
Andrew Chadwick hit 94, Graham Attenborough and Paul Goodwin 65 as Doncaster posted a good target, and while they restricted Driffield to 182-8 they could not dislodge the final two wickets.
Graham Attenborough ended the innings with 3-40 while Paul Stokoe and Luke Townsend both took two wickets.
Rotherham Town won by three wickets over Sheffield Collegiate with Yorkshire wicket keeper Simon Guy scoring an unbeaten 89 runs to underpin their innings of 192-7.
Collegiate had batted first and scored 190 all out, with Nick Gaywood's 78 being their only major contribution.
Greg Norton took 4-22 for Rotherham while Himesh Silva and Nathan Smith took two apiece.
In reply Rotherham needed 48 overs to reach a winning 192 with Guy making the lion's share while Alex Welsh also added 24.
Rotherham travel to Harrogate on Saturday, Doncaster take on fourth place York at Town Fields and Barnsley will look to extend their lead further as they welcome struggling Driffield.
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