A reform movement in the United States has focused on STEM education and 21st century soft skills such as critical thinking, communication, collaboration, and creativity. This spotlight on STEM instruction provided an opportunity to explore how K-14 STEM teacher participants perceived a Design Thinking Instructional Problems (DTIP) approach to developing instructional lessons. The study used a convergent parallel mixed-methods design with a survey instrument and a multiple case study focused on K-14 in-service STEM teachers. Data were collected from teacher participants during two five-week summer Research Experience for Teachers (RET) programs as part of two separate National Science Foundation (NSF) funded Engineering Research Centers (ERC) located at a large southwestern university in the United States (n=16). The study was conducted over three phases. During Phase I and II, teacher participants experienced a Design Thinking Overview workshop and weekly DTIP professional development sessions to facilitate the development of an RET instructional lesson. Pre- and post-program DTIP surveys and background interviews were conducted with all teacher participants (n=16). From this original group, teacher participants were selected as cases. Implementation observations and post-implementation interviews were conducted with these case-teachers (n=10). The study included frequency analysis and descriptive statistics of survey data. Qualitative data were analyzed using direct interpretation, thematic analysis, and open coding with the constant comparative method. A variety of arrays, summaries, and matrices were used to visualize patterns across and within individual case-teacher results. All 16 teacher participants viewed themselves as designers solving complex instructional problems. All 16 teacher participants found the DTIP professional development sessions to have somewhat to very much provided additional value during their RET summer programs. Six of the 10 case-teachers perceived the DTIP model graphic as mostly to completely corresponding to the way in which they developed their RET instructional lesson. Lastly, eight of the 10 case-teachers chose to embed a Design Thinking student learning strategy into the RET instructional lesson they developed.