Photosensitizer delivery is prerequisite for successful photodynamic therapy (PDT), and many photosensitizers have been developed and investigated in in vitro and in vivo studies 1. In general, hydrophobic photosensitizers can efficiently interact with biological components including cell membrane via hydrophobic interaction and exhibit high cellular uptake, inducing strong PDT effects in in vitro conditions. However, in in vivo conditions, such hydrophobic photosensitizers also interact with normal cells and tissues as well as target tumours, which causes unfavourable photochemical damage. On the other hand, hydrophilic photosensitizers avoid unfavourable interaction with biological components and their retention in the skin, reducing photosensitivity which is one of the side effects in PDT. However, the compromised interaction leads to low cellular uptake and PDT effects. Thus, it is difficult to develop photosensitizers that can induce strong PDT effects without showing severe photochemical damage to normal tissues. In this study, to integrate these conflicting properties of hydrophobic/hydrophilic photosensitizers, we developed a functional polymer exerting isothermal hydrophilic-to-hydrophobic phase transition in response to mildly acidic pH in tumours 2. The backbone of the polymer was a poly(N-isopropylacrylamide) derivative, which is well known to show lower critical solution temperature, and its side chain was modified with hydrophilic pH-cleavable moieties. The polymer termed P(NIPAAm/AIPAAm-PMM) showed hydrophilicity in a physiological condition (37°C, pH 7.4); however, in a tumour microenvironment-like condition (37°C, pH ≤ 6.9), P(NIPAAm/AIPAAm-PMM) exhibited hydrophobicity by detaching the hydrophilic moieties from the side chain. Owing to this pH-responsive hydrophilic-to-hydrophobic phase transition, the polymer exerted efficient cellular uptake in a pH-responsive manner. We then conjugated a phthalocyanine-based photosensitizer (IRDye 700DX) with the polymer and examined the potential of the polymer for PDT. In in vitro study, The photosensitizer-polymer conjugate exhibited strong PDT effects at acidic pH because of the aforementioned pH-responsive cellular uptake. In in vivo study, the photosensitizerpolymer conjugate efficiently accumulated within tumours in mice after intravenous injection and accomplished significantly enhanced PDT effects. Our results indicate that the control of hydrophilicity and hydrophobicity may be a promising approach to develop new photosensitizers for successful PDT.