Objective of the study: Analyze how Horizontal Innovation enables a company to produce radical and incremental innovations, making it ambidextrous.Methodology/approach: The research is a case study at a mature Brazilian company that puts Horizontal Innovation based on its Employee Suggestion System (ESS) into practice. A total of 18 interviews were conducted, including: all 4 directors and all 10 managers of the organization, in addition to 2 coordinators and 2 researchers.Originality/Relevance: The paper creates and defines the concept of Horizontal Innovation: a type of innovation that originates from all employees, usually stemming from an ESS. The study shows, based on continual ESS operations, how a company can achieve ambidexterity and systematically produce incremental, radical products and processes.Main results: ESS produces radical innovations as well as incremental ones, although the latter occur more frequently, and lead to Horizontal Innovations. The systematic search for Horizontal Innovations paves the way for a company to become ambidextrous, promoting, with the same competence, these two types of innovation.Theoretical/methodological contributions: The study contributes to the debate that defends the viability of innovative ambidexterity, and to new insights that clarify how a culture that stimulates ambidexterity can influence the ambidextrous behavior of employees through Horizontal InnovationPractical implications: A new type of innovation is proposed and described, reinforcing the value of an ESS as an important management tool. The case enables comparisons and contrasts with others for best practices benchmarking.