To answer this question can guinea pigs eat grapes the answer is yes, but in moderation! It is very important to note that grapes like many other fruits are very high in sugar and therefore need to be limited in a guinea pigs diet. Too much sugar can cause health issues such as digestive problems, obesity and possibly cause diabetes.
On the other hand the good thing about grapes for guinea pigs is that they are also high in vitamin C which are great for guinea pigs as they need this vitamin as part of their diet.
Therefore it is okay to give seedless grapes to your guinea pig in small amounts as a treat but nothing more than that. Make sure they are seedless grapes and they are washed and cut into small pieces. You want to avoid choking hazards.
Remember that even though your guinea pig is cute and will love to eat grapes it is important to stick with moderation when it comes to fruit. Too much will cause them to have long term health issues.
Therefore again the answer is yes guinea pigs can eat grapes but a better staple for their diet should include timothy hay, guinea pig pellets and fresh water. These foods should be given to guinea pigs on a daily basis.
If you are unsure of the types of grapes you should be feeding to your guinea pig make sure to speak with a vet or a local pet shop employee to get more details. If you are still unsure avoid feeding grapes all together to your guinea pig.