Poly(lactic acid) or polylactide (PLA) is a thermoplastic aliphatic polyester derived from renewable resources, such as corn starch (in the United States), tapioca products (roots, chips or starch mostly in Asia) or sugarcanes (in the rest of world). It can biodegrade under certain conditions, such as the presence of oxygen, and is difficult to recycle.

The name "polylactic acid" is to be used with caution, not complying to standard nomenclatures (such as IUPAC) and potentially leading to ambiguity (PLA is not a polyacid (polyelectrolyte), but rather a polyester)^{[citation needed]}.

Bacterial fermentation is used to produce lactic acid from corn starch or cane sugar. However, lactic acid cannot be directly polymerized to a useful product, because each polymerization reaction generates one molecule of water, the presence of which degrades the forming polymer chain to the point that only very low molecular weights are observed. Instead, two lactic acid molecules undergo a single esterfication and then catalytically cyclized to make a cyclic lactide ester. Although dimerization also generates water, it can be separated prior to polymerization due to a significant drop in polarity. PLA of high molecular weight is produced from the dilactate ester by ring-opening polymerization using most commonly a stannous octoate^{[citation needed]}catalyst, but for laboratory demonstrations tin(II) chloride is often employed. This mechanism does not generate additional water, and hence, a wide range of molecular weights is accessible.