This study investigates the geotechnical properties of soft Pak Phanang marine clay, prevalent in Nakhon Si Thammarat province, Thailand, where rapid economic development demands a comprehensive understanding for sustainable construction. Triaxial tests on undisturbed marine clay specimens with various stress histories and strain rates were conducted, focusing on over-consolidation ratios (OCRs) of 1, 2, 4, and 8. Shearing was performed at rates of 0.020%, 0.075%, 1.000%, and 8.500% per minute after K0 consolidation. The strain rates selected for this study represent specific values that have been chosen for a comprehensive exploration of Pak Phanang clay behavior under different loading conditions. The effects of stress histories on the marine clay behavior at various strain rates under K0 conditions were investigated. It is indicated that the greater strain rates under K0 conditions potentially lead to the larger undrained shear strengths and reduce pore water pressure for varied over-consolidation ratios. On the other hand, the greater over-consolidation ratios commonly result in lower shear strengths at all strain rates. Examination of pore pressure parameter at failure (Af\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${A}_{f}$$\end{document}) and secant Young's modulus reveals significant strain-rate-dependent behavior and OCR influence on the marine clay's response. Undrained shear strength increases with higher OCRs, emphasizing OCR's pivotal role. Rate effect analysis confirms undrained behavior, with a consistent 28% strength increase, regardless of OCR variations. Pore pressure responses exhibit a transition at OCR 4. Secant Young's modulus decreases with rising OCR, establishing a linear correlation with undrained shear strength.