This study was primarily aimed at developing an English-speaking proficiency test and analytic rubrics designed to measure speaking proficiency of Malaysian undergraduates. On the basis of Littlewood's Methodological Framework and Long's Interaction Hypothesis, the researchers derived three speaking tasks from four sources: (a) syllabus of the English language courses at the relevant university, (b) Kathleen Bardovi-Harlig's operationalizing conversation speech acts, (c) IELTS part B speaking test, and (d) task B speaking section of Malaysian University English Test (MUET). A total of 96 undergraduates with four levels of the language proficiency (i.e., low performers, intermediate performers, upper-intermediate performers, and high performers) from a public university in Malaysia voluntarily participated in the study. While two TESOL experts were invited to validate the content of the tasks and the rubrics, two raters rated students' test scores. Construct validity was established through a known-group validity (construct validity) for a known-group comparison of the task performance at the three difficulty levels namely, elementary, intermediate and advanced. The test scores, having good internal consistency (a= 0.89) and inter-rater reliability (ICC= 0.84), yielded speaking proficiency descriptors. This result showed that the test is reliable and valid to diagnose speaking proficiency of Malaysian undergraduates in pursuit of improvement.