Recently, the question of which version of content knowledge (CK) is suitable for pre-service teacher (PST) development has gained prominence in teacher education research. A version of CK argued to be appropriate for science PST development has been introduced and referred to as teacher-related science content knowledge (TerSCK). This CK version is distinct from academic discipline and school subject CK in science. However, TerSCK plays a relational role between the two. Without valid and reliable instruments to measure this version of CK, this study used the case of organic chemistry to design a valid and reliable instrument to measure this version of CK. A mixed-methods research approach was followed with 35 PSTs who were purposively sampled. A reference team of experts in the field was also set up to provide insights into the judgement of items and to ensure the instrument's content and face validity. Data collected through the designed instrument was analysed using Rasch statistical software. Findings indicated an acceptable range of fit statistics of 0.5-1.5 mean-square values and -2 to +2 standardised values confirming the validity of the instrument. The reliability indices were also found to be good at 0.65 and 0.93 for person and item reliability indices, respectively. This paper further discusses the implications that this instrument has for teacher development. Even though the findings were limited in scope, the study serves as a baseline for further studies to develop and validate such instruments for measuring CK for teaching science in other topics.