Graffiti is one of the salient and less recognized deviant behavours in schools and among the research scholars, particularly in Nigeria. In view of this, this study investigated the patterns of graffiti behaviour among in-school adolescents in Ilorin metropolis, Kwara State. The study followed the descriptive survey design; using a proportionate sampling technique to select 390 respondents. The instrument used was a self-designed "Patterns of Graffiti Behaviour Questionnaire" with Cronbach reliability coefficient of 0.81. About 74 copies of the questionnaire were not returned; while 126 copies were wrongly filled; hence, 200 copies of the questionnaire were included in data analysis. Percentage, t-test and Analysis of Variance (ANOVA) statistics were the data analyses tools employed in this study. The results showed that inscription on the school toilet walls, drawing sex organ on toilet walls and drawing of a female's breast on school wall to communicate sexual harassment are the topmost patterns of graffiti behaviour among in-school adolescents in Ilorin metropolis. The hypotheses tested indicated that (gender, age and family; p=0.10; 0.08 and 0.16>0.05) have no significant influence on the patterns of graffiti behaviour among the respondents. Sequel to the outcome of this research, relevant recommendations were provided.