Pre-trained language models have recently advanced abstractive summarization. These models are further fine-tuned on human-written references before summary generation in test time. In this work, we propose the first application of transductive learning to summarization. In this paradigm, a model can learn from the test set's input before inference. To perform transduction, we propose to utilize input document summarizing sentences to construct references for learning in test time. These sentences are often compressed and fused to form abstractive summaries and provide omitted details and additional context to the reader. We show that our approach yields state-of-the-art results on CNN/DM and NYT datasets. For instance, we achieve over 1 ROUGE-L point improvement on CNN/DM. Further, we show the benefits of transduction from older to more recent news. Finally, through human and automatic evaluation, we show that our summaries become more abstractive and coherent.