The adjectival morphology of Classical Latin largely replicated the nominal morphology of the three main noun classes or declensions. In other words, the adjective was inflected not just for gender and number, as it is in modern Spanish, but also for case.

Some adjectives took their endings from both the first and second nominal declensions (with accusative singular in -am and -um respectively), depending on whether the noun they modified was feminine or masculine/neuter, while others took their endings exclusively from the third declension (with singular accusative forms in -em).

Over time, the adjectival system underwent essentially the same fate as the nominal system. In other words, all case distinctions were eventually eliminated, with only the accusative form (now devoid of any specific case value) surviving into Spanish, and the neuter gender was entirely lost.

Thus the morphology of the erstwhile first/second-declension group of adjectives became reduced to a masculine singular ending in /-o/ (representing a continuation of the Classical -um ending), a feminine singular ending in /-a/ (continuing the Classical -am ending) and a plural in -s (a continuation of the old accusative plural):

frío, fría, fríos, frías ‘cold’ < frīgidum, frīgida, frīgidōs, frīgidās

Analogously, the erstwhile third-declension group of adjectives lived on with a fixed singular ending in -e (continuing the Classical -em ending) and a plural form in -s (continuing the old accusative plural):

alegre, alegres ‘happy’ < alacrem, alacrēs
pobre, pobres ‘poor’ < paup(e)rem, paup(e)rēs

Where the final -e in this type of adjective occurred immediately after an ungrouped dental or alveolar consonant it was regularly lost through apocope in medieval Spanish, resulting in modern consonant-final singular forms such as the following:

igual ‘same’ < *iguale < aequālem

As happened in the case of the noun, gender hypercharacterization produced a certain amount of restructuring during the medieval period. In particular, adjectives with singular forms in -or (< -ōrem) or -ón (< -ōnem), together with some in -és (< -ēnsem), which hitherto were unmarked for gender, acquired bespoke feminine forms in the late Middle Ages and early modern period:

traidora < traidor ‘treacherous’ < trāditōrem
Old Sp. espannona < espannon ‘Spanish’ (later: español, -a under Gallic influence)
Old Sp. burgesa < burges ‘citizen of a town’ (later: burgués, -a)

Note, however, montés ‘wild’ and cortés ‘polite’, which have no feminine form in modern standard Spanish.

Except in a handful of cases, the Classical Latin inflected comparative and superlative forms, such as grandior ‘bigger’ and grandissimus ‘biggest/very big’, gave way in Vulgar Latin to constructions involving the adverbs magis ‘more’, māxime ‘most’ and mult(um) ‘very’; thus magis grandis ‘bigger’, māximē grandis ‘biggest’, mult(um) grandis ‘very big’. (An alternative existed for the comparative, involving the adverb plūs, but this does not seem to have taken root in the area where Spanish developed.) Māximē was soon abandoned but reflexes persist of the magis and mult(um) constructions: más listo ‘cleverer’, el más listo ‘the cleverest’, muy listo ‘very clever’ (in a different syntactic role, of course, multum also yields the adverb mucho ‘a lot’). A few very common comparative forms survived, namely mejor ‘better’ (< meliōrem), peor ‘worse’ (< pēiōrem), mayor ‘older/greater’ (< māiōrem) and menor ‘lesser’ (< minōrem). These were not subject to gender hypercharacterization.

The suffix -ísimo, as in guapísimo ‘very handsome’, rarísimo ‘very strange’, is a cultismo or learned borrowing from Latin rather than a genuine continuation of the Latin superlative.