Williams has played every minute of Swansea's Barclays Premier League campaign and missed only seven games during the Welsh club's five seasons in the top flight
The 31-year-old was also an ever-present in Wales' successful Euro 2016 qualifying campaign, and Guidolin feels the central defender is due a rest after Swansea secured their top-flight safety with a 3-1 victory over Liverpool last weekend
Guidolin's decision will delight Wales manager Chris Coleman, who has acknowledged the amount of football his skipper has played over the last few seasons.
"Ash will not play again this season, he needs a break," said Guidolin ahead of Saturday's trip to West Ham.
"He is tired because he has played a lot of games this season and before.
"Ash is the skipper, a good player and leader, and he is very important for me and our team.
"But it is better for Ash to rest and be prepared for the Euros and for next season."
Wales defender Neil Taylor and Gylfi Sigurdsson will also miss the trip to West Ham and Swansea's final game of the season, at home to Manchester City.
But Guidolin says their absences are down to injuries which require treatment, although it is understood that their participation at Euro 2016 is not in doubt
Jefferson Montero is also missing, but Guidolin has promised West Ham's rivals for European qualification that Swansea are still capable of causing a shock at what will be the Boleyn Ground's penultimate game before the Hammers move to the Olympic Stadium.
"West Ham is a very good team and they have had a good season," said Guidolin.
"But I am confident that we have a good group, it is not not only 11 players
"I have the chance to see some players who don't play much and this is important for the future of our club and our players."
Whether Guidolin, who was appointed in January and told his position would be reviewed at the end of the season, will be part of Swansea's future remains to be seen.
The Italian says he spoke to chairman Huw Jenkins on Wednesday and expects his future to be resolved after the City game on May 15.
"We achieved our target (of staying up) and now the atmosphere is nice," said Guidolin.
"We are more relaxed and the performance in general in these four months has been good.
"I am proud of my players but we are not finished, we have two more games against big clubs."
Source : PA
Source: PA