Michael Clarke struck a sparkling hundred as Australia beat Pakistan by eight wickets in the fourth day-night international in Abu Dhabi on Friday to take an unassailable 3-1 lead in the series.
The 28-year-old Australian captain hit an unbeaten 100 and was well supported by Watson (85 not out) as the pair added 197 for the third wicket to ease Australia past a modest 198-run target with 5.4 overs to spare.
Pakistan were bowled out for 197 in 48.4 overs with Australian left-arm paceman Doug Bollinger taking 5-35 in only his second international one-day match.
Pakistan won the first match by four wickets before Australia came back strongly to win the second by six wickets and the third by 27 runs. The fifth and final match will be played in Abu Dhabi on Sunday.
This is Australia's first one-day series win since they beat Bangladesh last September, having lost to South Africa home and away and drawn 2-2 with New Zealand before sealing this series win over Pakistan.
Pakistan paceman Shoaib Akhtar removed Brad Haddin (nought) and then debutant Marcus North with only three runs on the board, before Clarke and Watson came to Australia's rescue and put on a record stand against Pakistan.