The study investigated the effectiveness of teacher-guided inquiry technique on learning outcomes of senior secondary school students in Geography in Lagos State, Nigeria. Four research questions were answered and four hypotheses were tested at a 0.05 level of significance. The study adopted a quasi-experimental pretest posttest non-equivalent control group design. The population of the study consisted of all senior secondary two (SS 2) Geography students in Lagos State. Multistage sampling techniques were used to narrow down the sample to four (4) intact classes of students who were then selected purposively to give a sample size of 99. Geography Achievement Test (GAT) and Geography Students’ Attitude Questionnaire (GSAQ) were used for data collection. The internal consistencies of the instruments were measured using Cronbach’s alpha with co-efficient values of 0.76 for GAT (pre and post-test); 0.87 for GSAQ pre-treatment and 0.79 for the GSAQ post-treatment. While mean and standard deviation were used to answer the research questions, an independent t-test was used to test all the hypotheses at a 0.05 level of significance. The findings revealed that teacher-guided inquiry technique is more effective in improving academic achievement and attitude of students toward  Geography than the use of conventional method and that, there was no significant difference in the academic achievement and attitude of students based on gender. Based on the findings of the study, it was recommended among others that curriculum planners and teacher training institutions in Nigeria should revisit their curriculum to embrace more inquiry-based techniques with less emphasis on conventional methods; workshops, seminars, and conferences should be organized to enlighten Geography teachers on the applications of teacher-guided inquiry technique in classroom teaching; the school time-table should be restructured to give more opportunity for the practice of teacher-guided inquiry technique, and above all, gender stereotyping should have no place in Geography classroom.