This paper introduces a novel hashtag graph-based topic model (HGTM) for analyzing microblogging environments, specifically to address the challenges posed by the informal and sparse nature of tweets. By utilizing user-contributed hashtags, HGTM connects semantically related words even when they do not co-occur, thus enhancing topic discovery and alleviating issues related to sparseness and noise in tweet data. Experimental evaluations demonstrate that HGTM outperforms traditional topic models and achieves effective clustering and classification on real-world tweet datasets.