Prompt-OVD is an efficient and effective DETR-based framework for open -vocabulary object detection that utilizes class embeddings from CLIP as prompts, guiding the Transformer decoder to detect objects in base and novel classes. Additionally, our RoI-pruned masked attention helps leverage the zero -shot classification ability of the Vision Transformer -based CLIP, resulting in improved detection performance at a minimal computational cost. Our experiments on the OV-COCO and OV-LVIS datasets demonstrate that Prompt-OVD achieves an impressive 21.2 times faster inference speed than the first end -to -end open -vocabulary detection method (OVDETR), while also achieving higher APs than four two -stage methods operating within similar inference time ranges. We release the code at https://github.com/DISL-Lab/Prompt-OVD.