Pedagogical approaches for supporting students' argumentative writing in science laboratories have not been fully established. This article examines the development of argumentative abilities in undergraduate students enrolled in chemistry laboratory courses that employed two teaching sequences: (i) an argumentative writing workshop for conceptual learning and (ii) weekly laboratory report writing for application of knowledge gained in the workshop. The four workshop modules guided students through the process of identifying three key components of arguments (evidence, justifications, and claims), selecting appropriate and inappropriate justifications, constructing justifications and conclusions, and analyzing experimental errors. Student performance in formulating scientific arguments was evaluated through instructors' assessment of evidence, justifications, and claims made in the Results, Discussion, and Conclusions sections, respectively, of a laboratory report. Student performance improved from 60.9 ± 3.4 to 91.5 ± 8.0 in Introductory Chemistry I Lab and 60.7 ± 5.2 to 91.7 ± 5.4 in Introductory Chemistry II Lab. Students rated the helpfulness of the writing workshop ([3.6 ± 0.1]/5.0), weekly writing ([4.1 ± 0.3]/5.0), and instructors' feedback ([4.4 ± 0.5]/5.0) for both introductory and advanced chemistry laboratories positively. The format of this writing workshop can be used for online teaching or incorporated into any science laboratory course with the development of appropriate content modules.