People are posting huge amounts of varied information on the Web as the popularity of social media continues to increase. The sentiment of a tweet posted on Twitter can reveal valuable information on the reputation of various targets both on the Web and in the real world. We propose a method to classify tweet sentiments by machine learning. In most cases, machine learning requires a significant amount of manually labeled data. Our method is different in that we use social bookmark data as training data for classifying tweets with URLs. In social bookmarks, comments are written using casual expressions, similar to tweets. Since tags in social bookmarks partly represent sentiment, they can be used as supervisory signals for learning. The proposed method moves beyond the basic “positive”/“negative” classification to classify impressions as “useful”, “funny”, “negative”, and “other”.