This study was carried out to investigate the Online Information Seeking Behaviour of Postgraduate Students in Ahmadu Bello University, Zaria. Five research questions were raised and five (5) hypothesis were formulated and tested at α=0.05. The research questions sought to find out the types of online information resources and services available for use by postgraduate students, their sources of online information, their information search pattern, how they evaluate and filter the quality of online information resources (OIRs) they use. The research adopted survey method; the instrument used for data collection was the questionnaire. Proportionate stratified random sampling was used to select respondents for this study. Data collected from the research questions were analysed using frequency tables and simple percentages, Pie Chart and Column Charts. Null hypotheses 1, 3, 4 and 5 were analysed using ANOVA while Null hypothesis 2 was analysed using t-test. The study discovered that majority of the postgraduate students are aware of the online information resource which they frequently use. They are also aware of e-journals, e-books and electronic theses and dissertations; majority of the postgraduate students in Ahmadu Bello University, Zaria are extraverted and enthusiastic by the results and amount of information they get online. This behavior affects their ability to get the required information; There is no significant difference between male and female students in Ahmadu Bello University on the sources of online information resources available for their use; There is no significant difference among students of different faculties of Ahmadu Bello University concerning the challenges faced in searching for online information resources; majority of the postgraduate students use Google and other search engines to look up information online. The study concluded that postgraduate students in Ahmadu Bello University have realized and have come to terms with the web knowing fully well that it is packed full with resources that they need for their academic advancement and research productivity. However, many of the postgraduate students have developed poor online information seeking behavior making it difficult for them to locate the right information. It is recommended that more awareness should be created on the availability Ahmadu Bello University's institutional repository and how to effectively search for information therein; the University Library should intensify the existing training programme for postgraduate students on how to effectively search for online information resources.