Soluble sugars in feeds are important for ruminant production; however, performing numerous sugar analyses within a short period is a laborious task. Here, we developed a phenol‐sulfuric acid (PSA) assay in a microplate format to quantify soluble sugars in ruminant feeds. This method is easy and quick and requires only a small quantity of harmful reagents. We found that assay measurements were not affected by the representative organic acids and sugar alcohol contained in feeds. The treatment of activated charcoal with ethanol extract prior to the PSA assay was effective in removing interfering compounds for a more accurate determination of soluble sugars in certain feeds. Furthermore, the inter‐day and intra‐day repeatability of the present method was acceptable. Hence, we conclude that the method developed in this study is suitable for routine analysis of soluble sugars content in ruminant feeds.