Leaves opposite or in whorls of 3 or 4, simple, margins mostly entire; stipules interpetiolar, free or fused, sometimes leaf-like and giving the appearance of whorled leaves.
Inflorescence a thyrse or dichasium, rarely flowers fused into a compound head or a single, terminal flower. Flowers usually actinomorphic, bisexual or unisexual and then plants monoecious or dioecious, mostly 4- or 5-merous. Calyx 3–6-lobed, often rudimentary or absent. Corolla mostly tubular and fused with the filaments at least at base, usually 3–5-lobed, sometimes ± 2-lipped. Stamens 4 or 5, alternate with petals; anthers 2-locular, dehiscing by longitudinal slits. Ovary inferior, 1- or more-locular; ovules 1–many per loculus; style undivided or bifid.
Fruit a capsule, drupe or schizocarp, individual fruit often fused into a compound fruit.
| | Key to the family RUBIACEAE | |
| 1 | Herbs mostly less than 1 m high, or if shrubby then with an inflorescence of globose compound heads | 2 |
| 2 | Leaves and stipules similar, giving the appearance of whorled leaves | 3 |
| 3 | Flowers in heads; calyx present, lobed | Sherardia |
| 3* | Flowers separate; calyx usually absent or present and then not lobed | 4 |
| 4 | Corolla rotate, tube very short; fruit of 2 dry 1-seeded mericarps, falling separately at maturity | Galium |
| 4* | Corolla funnel-shaped with a distinct tube; fruit fleshy, 2-lobed, indehiscent | Asperula |
| 2* | Leaves and stipules dissimilar, latter much smaller | 5 |
| 5 | Flowers united into simple or compound heads by fusion of inferior ovaries | 6 |
| 6 | Flowers joined in 2- or 3-flowered heads, 5–11 heads on umbellate stalks | Pomax |
| 6* | Flowers joined in 2–7-flowered heads, several heads together in a compound globose head | Opercularia |
| 5* | Flowers not united into heads by fusion of ovaries | 7 |
| 7 | Inflorescences terminal, or if axillary then leafy bracts surrounding inflorescence | 8 |
| 8 | Flowers solitary | 9 |
| 9 | Calyx lobes absent; corolla short-campanulate | Nertera |
| 9* | Calyx lobes well-developed; corolla tubular to funnel-form or trumpet-shaped | Leptostigma |
| 8* | Flowers few to many in cymes or heads | 10 |
| 10 | Flowers in heads or clusters, surrounded by leafy bracts | 11 |
| 11 | Plants erect; fruit of only 2, 1-seeded carpels, crowned by 2–4 persistent, erect calyx lobes | Borreria |
| 11* | Plants prostrate; fruit separating into 3 or 4 one-seeded mericarps, crowned by 4–6 eventually falling, radiating calyx lobes | Richardia |
| 10* | Flowers in cymes, not surrounded by leafy bracts | 12 |
| 12 | Flowers bisexual; cymes regular; ovary 2-locular; fruit dry | Knoxia |
| 12* | Flowers unisexual and plants dioecious; cymes arranged in compound dichasia; ovary 1-locular; fruit fleshy | Durringtonia |
| 7* | Inflorescences axillary; leafy bracts never present | 13 |
| 13 | Non-succulent herbs of variable habit with stems to 40 cm long; stipules with long cilia | Diodia |
| 13* | Succulent prostrate herbs with stems to 20 cm long; stipules not as above | 14 |
| 14 | Leaves less than 4 mm long; flowers sessile | Dentella |
| 14* | Leaves mostly more than 4 mm long; flowers pedicellate | 15 |
| 15 | Corolla with a short or long tube, valvate, not persistent in fruit; pedicels to 20 mm long | Oldenlandia |
| 15* | Corolla rotate, persistent in fruit; pedicels to 6 mm long | Synaptantha |
| 1* | Woody plants, usually more than 1 m high | 16 |
| 16 | Climbing plants | 17 |
| 17 | Flowers fused, at least in part; fruit a syncarp | Morinda |
| 17* | Flowers free; fruit a drupe with 4 distinct 1-seeded pyrenes | Caelospermum |
| 16* | Shrubs or trees | 18 |
| 18 | Ovaries with 1–5 per ovules per loculus | 19 |
| 19 | Stipules fused, at least at base | 20 |
| 20 | Style exserted from corolla tube, the exserted part much longer than corolla lobes | Pavetta |
| 20* | Style slightly exserted from corolla tube, the exserted part shorter than or as long as corolla lobes | Ixora |
| 19* | Stipules free | 21 |
| 21 | Corolla lobes valvate in bud | 22 |
| 22 | Flowers along leafy branchlets | 23 |
| 23 | Flowers axillary, in clusters or cymes, rarely solitary; ovules attached at or near top of ovary | Canthium |
| 23* | Flowers terminal or terminal and axillary in upper leaf axils, usually united into terminal inflorescences; ovules attached at base of ovary | Psychotria |
| 22* | Flowers along branches behind leaves or on axillary flower-bearing short shoots | Coprosma |
| 21* | Corolla lobes contorted and/or imbricate | 24 |
| 24 | Flowers unisexual in umbels on slender axillary peduncles | Hodgkinsonia |
| 24* | Flowers bisexual in clusters or dense cymes | Diplospora |
| 18* | Ovaries with at least 6 ovules per loculus | Randia |
