Writing is one of the most difficult skills for the learners of English as a foreign language (EFL) as it involves the combination of linguistic knowledge, cognitive organization and communication skills. Iraqi EFL preparatory school learners are likely to experience problems in idea generation, structuring written texts, and in collaborative writing activities. Hence, the present study aimed to examine the effect of Structured Sequential Writing Procedure on the collaborative writing competence of Iraqi EFL fourth preparatory school students. The study was designed as quasi-experimental with a pretest–posttest control group design. The experiment was carried out from October 15, 2025, to January 7, 2026. Sixty learners from Al-Imam Ali Preparatory School in Babylon Governorate, Iraq, participated in the study and were equally assigned to an experimental group and a control group. The experimental group was instructed using the Structured Sequential Writing Procedure while the control group was instructed using the traditional method. A collaborative writing achievement test was given as a pretest and posttest. Cronbach's alpha was used to determine the reliability of the instrument, and the results were .79 and .89. The results showed no statistically significant differences between the two groups in the pretest. The results of the posttest, however, showed a statistically significant difference in favor of the experimental group, t(58) = 7.09, p < .05. The effect size was large (Cohen's d = 1.83) which showed the effectiveness of the instructional procedure in improving the learners' collaborative writing competence.