Wales move two slots the other way to No.10, their lowest position since the rankings started in October 2003.
The top five places are unchanged, with New Zealand ahead of Australia, South Africa, Argentina and France.
But the Pumas have closed the gap on the Springboks to less than one rating point following their win over Ireland on Sunday, which confirmed them as Pool D winners and secured a quarter-final match with Scotland in Paris on Sunday.
Other movers include England, up one spot to No.6, and Scotland up two places to No.8, while Italy (11) and USA (17) move two places in the other direction. Fellow pool casualties Ireland also drop a place, to No.7.
RWC results count for double; teams gain more points for victories, and lose more when beaten.
The rankings are published every Monday at midday UK time on www.irb.com