Polymer-based nanomaterials can deliver antitumor reagents to tumor tissues effectively due to the enhanced permeation and retention (EPR) effect. To further improve the tumor delivery efficacy, a targeting moiety should be installed in the polymer building blocks. In this regard, an acidic pH is one of the characteristics of tumor sites, whereby polymers that respond to acidic conditions would realize the construction of tumor-targeted nanomaterials. In this review, we explain the rationale strategies for the design of functional polymers with responsiveness to a low tumor pH and describe examples of smart nanomaterials designed for selective tumor delivery.Acidic pH is identified for various types of tumors, whereby it can be employed for crafting tumor-targeted nanomaterials. Cationic net charge of the nanomaterials at tumorous pH achieves selective interaction with anionic tissue constituents at tumor sites, for the effective tumor accumulation. However, tumorous pH is ca. 6.5, whereas pH of normal tissues is 7.4, and therefore responsiveness to the small pH window is the key toward the success for tumor delivery. The present manuscript highlights the polymer designs that recognize tumorous pH to make tumor-targeted nanomaterials.