Decentralized Web Applications (dApps) built on blockchain, Web3 technologies, and the Inter- Planetary File System (IPFS) are emerging as a promising solution to longstanding challenges in agriculture. Conventional centralized systems often result in opaque supply chains, data tampering, fraud, and limited empowerment of smallholder farmers. This systematic review identifies and analyzes 12 representative studies published between 2017 and 2025, selected via a structured search across IEEE Xplore, Scopus, and Google Scholar using a defined inclusion and exclusion protocol. Studies are examined with particular emphasis on supply-chain traceability, IoT-enabled smart farming, parametric crop insurance, direct farmer-to-buyer marketplaces, and secure farm-data management. Most implementations leverage Ethereum smart contracts or Hyperledger Fabric, integrate IoT sensors for real-time monitoring, and employ IPFS for off-chain storage of large files such as sensor readings and images. Key benefits include immutable records that prevent tampering, end-to-end traceability for rapid identification of contaminated produce, automatic smart-contract payments, and trust-building without intermediaries. Notable examples are the Walmart-IBM blockchain pilot for mango and pork traceability and platforms such as Etherisc and Arbol for parametric crop insurance. However, challenges remain, including high gas fees and slow transaction speeds on public blockchains, high energy consumption, interoperability issues, data privacy concerns, and limited digital infrastructure among smallholders in regions such as India. This review synthesizes findings across four core application areas—data storage, supply-chain tracking, smart-contract automation, and security/trust—and identifies six open research gaps. It concludes that dApps have strong potential to make agriculture more transparent, equitable, and sustainable, provided that scalability, usability, and regulatory barriers are addressed.