Managing multiple projects simultaneously is the current trend in project management particularly in the oil and gas industry. The objective of this research is to investigate the impact of teamwork quality on project effectiveness in a multiple-project management setting in order to help project managers manage multibillion dollar projects effectively. Teamwork quality is measured against six variables: communication, coordination, balance of member contributions, mutual support, effort and cohesion, while effectiveness is measured by project performance. A total of 184 project managers participated in the survey. The findings show that all six variables of the teamwork quality construct had a measurable impact on project performance.