Compared to conventional general-purpose processors, accelerator-rich architectures (ARAs) can provide orders-of-magnitude performance and energy gains. In this paper we design and implement the ARAPrototyper to enable rapid design space explorations for ARAs in real silicons and reduce the tedious prototyping efforts. First, ARAPrototyper provides a reusable baseline prototype with a highly customizable memory system, including interconnect between accelerators and buffers, interconnect between buffers and last-level cache (LLC) or DRAM, coherency choice at LLC or DRAM, and address translation support. To provide more insights into performance analysis, ARAPrototyper adds several performance counters on the accelerator side and leverages existing performance counters on the CPU side. Second, ARAPrototyper provides a clean interface to quickly integrate a user?s own accelerators written in high-level synthesis (HLS) code. Then, an ARA prototype can be automatically generated and mapped to a Xilinx Zynq SoC. To quickly develop applications that run seamlessly on the ARA prototype, ARAPrototyper provides a system software stack and abstracts the accelerators as software libraries for application developers. Our results demonstrate that ARAPrototyper enables a wide range of design space explorations for ARAs at manageable prototyping efforts and 4,000 to 10,000X faster evaluation time than full-system simulations. We believe that ARAPrototyper can be an attractive alternative for ARA design and evaluation.