Photodynamic therapy (PDT) is a rapidly evolving approach based on the elimination of target cells with light-activated photosensitizers (PS). The main limitation of PDT is the low penetration of visible light used to activate PS. Upconversion nanoparticles (UCNPs) provide a unique platform for the development of systems based on deep-penetrating near-infrared (NIR) light due to their ability to absorb NIR light and emit visible light capable of activating PS. However, UCNPs are usually hydrophobic and therefore require surface hydrophilization and biofunctionalization. In this work, we have developed a one-step technique to form a NIR-activated PDT nanocomplex consisting of NaYF:YbEr/NaYF particles functionalized with novel amphiphilic chlorin derivatives by hydrophobic self-assembly to address both light penetration and UCNP surface functionalization issues. To this end, three novel chlorophyll a derivatives based on chlorin p with a hydrophobic octadecylamine moiety and a hydrophilic PEG and PEG moieties were synthesized. Förster resonance energy transfer was demonstrated for these nanocomplexes, and one of them, namely the complex with compound 4 (octadecyl-Chlp-PEG) with 19% FRET efficiency, was selected for further investigation. The in vitro cytotoxicity of compound 4 was studied using murine fibroblast L-929, human fibroblast WI-26 and human breast adenocarcinoma MCF-7 cell lines, and a 3-fold higher phototoxicity index was found in comparison with the commercial Photoditazine (Chlorin е). The in vivo study using the Lewis Lung Cancer (LLC) model demonstrated 41.2% tumor growth inhibition efficacy compared to the control group on day 14 after irradiation. We conclude that NIR-activated nanocomplexes of UCNPs-compound 4, as well as water-soluble novel amphiphilic chlorophyll a derivatives, can be promising for PDT purposes. The obtained nanocomplex can be considered as the next generation of PSs which means the coupling of PSs with carriers for targeted delivery of antitumor drugs.