Pakistan completed a 5-0 series rout after posting a daunting 329/9, their highest total against Bangla, and skittling the visitors for 179 in 40.5 overs.
Butt's 136, his second hundred in the series, gave him the record for most runs (451) scored by a Pakistani in a bilateral series and by any one-day international batsman in a five-match series.
He comfortably surpassed the previous best of 405 by compatriot Mohammad Yousuf in a five-match series against Zimbabwe in 2003.
Pakistan has won 11 ODIs in as many games with 10 coming against Bangladesh and Zimbabwe.
Bangladesh's batsmen were undone by the new-ball on Saturday.
Mohammad Asif took three quick wickets, including two in successive balls in his third over, to reduce Bangladesh to 23 for four.
Captain Mohammad Ashraful made his highest score of the series (30) before his soft dismissal in the 14th over opened the floodgates again.
Earlier, Butt again put Bangladesh's bowlers to the sword, making his sixth one-day hundred from 100 balls with nine fours and one six.
He and Younis Khan (69) set the platform for the record total by putting on 179 from 178 balls for the second wicket after Kamran Akmal (10) fell to Mortaza, who finished with four for 65.