Pitchers had their best ERA in a postseason in the 1905 World Series, with an ERA of 0.83.

Interpreted as:

best MLB active postseason starter era in a season

	POSITION	ERA	SEASON	G	W	L	W%	GS	SO	CG	SHO	SV	IP	H	ER	R	HR	BB	HBP	GF	IBB	TBF	WP	K/9	BB/9	K/BB	HR/9	K%	BB%	WHIP	BABIP	RA	H/9
1	Pitcher	0.83	1905	10	5	5	.500	10	50	9	5	0	87.0	56	8	18	0	19	3	0	2	334	2	5.17	1.97	2.63	0.00	15.0	5.7	0.86	.214	1.86	5.79
2	Pitcher	1.17	1950	8	3	3	.500	8	31	3	1	0	61.2	45	8	12	1	19	1	0	2	247	1	4.52	2.77	1.63	0.15	12.6	7.7	1.04	.226	1.75	6.57
3	Pitcher	1.41	1918	12	6	5	.545	12	35	9	2	0	102.0	68	16	18	0	34	3	0	1	387	1	3.09	3.00	1.03	0.00	9.0	8.8	1.00	.216	1.59	6.00
4	Pitcher	1.45	1907	10	4	4	.500	10	45	8	1	0	87.0	76	14	24	0	21	5	0	0	351	0	4.66	2.17	2.14	0.00	12.8	6.0	1.11	.271	2.48	7.86
5	Pitcher	1.55	1966	8	3	4	.429	8	28	4	3	0	52.1	33	9	12	5	17	0	0	2	199	1	4.82	2.92	1.65	0.86	14.1	8.5	0.96	.188	2.06	5.68
6	Pitcher	1.59	1911	12	5	5	.500	12	67	7	0	0	96.0	68	17	31	3	18	3	0	1	373	2	6.28	1.69	3.72	0.28	18.0	4.8	0.90	.230	2.91	6.38
7	Pitcher	1.89	1915	10	5	4	.556	10	48	9	0	0	81.0	65	17	19	2	19	2	0	1	316	0	5.33	2.11	2.53	0.22	15.2	6.0	1.04	.257	2.11	7.22
8	Pitcher	1.92	1944	12	5	5	.500	12	69	5	1	0	79.2	66	17	24	4	30	0	0	3	336	1	7.79	3.39	2.30	0.45	20.5	8.9	1.21	.266	2.71	7.46
9	Pitcher	2.00	1920	14	7	7	.500	14	27	8	3	0	94.2	82	21	25	2	21	0	0	2	375	1	2.57	2.00	1.29	0.19	7.2	5.6	1.09	.246	2.38	7.80
10	Pitcher	2.02	1943	10	5	4	.556	10	51	4	1	0	80.1	63	18	25	4	16	0	0	4	321	2	5.71	1.79	3.19	0.45	15.9	5.0	0.98	.236	2.80	7.06
11	Pitcher	2.12	1963	8	4	4	.500	8	54	3	1	0	59.1	42	14	15	5	14	1	0	2	230	2	8.19	2.12	3.86	0.76	23.5	6.1	0.94	.237	2.28	6.37
12	Pitcher	2.13	1906	12	6	5	.545	12	50	8	2	0	84.2	53	20	31	0	28	4	0	1	337	5	5.31	2.98	1.79	0.00	14.8	8.3	0.96	.208	3.30	5.63
13	Pitcher	2.17	1930	12	5	6	.455	12	56	8	1	0	87.0	61	21	25	6	30	1	0	3	351	1	5.79	3.10	1.87	0.62	16.0	8.5	1.05	.213	2.59	6.31
14	Pitcher	2.25	1965	14	7	7	.500	14	66	7	3	0	92.0	82	23	31	9	18	1	0	0	372	1	6.46	1.76	3.67	0.88	17.7	4.8	1.09	.263	3.03	8.02
15	Pitcher	2.31	1912	16	6	7	.462	16	68	9	0	0	121.0	110	31	48	1	29	2	0	1	498	3	5.06	2.16	2.34	0.07	13.7	5.8	1.15	.274	3.57	8.18
16	Pitcher	2.31	1946	14	5	5	.500	14	45	6	2	0	93.1	83	24	34	4	25	3	0	4	383	0	4.34	2.41	1.80	0.39	11.7	6.5	1.16	.258	3.28	8.00
17	Pitcher	2.34	1921	16	6	6	.500	16	54	11	3	0	111.1	86	29	34	3	38	1	0	0	441	2	4.37	3.07	1.42	0.24	12.2	8.6	1.11	.241	2.75	6.95
18	Pitcher	2.35	1974	26	11	11	.500	26	103	4	3	0	168.2	122	44	51	15	72	5	0	5	689	5	5.50	3.84	1.43	0.80	14.9	10.4	1.15	.217	2.72	6.51
19	Pitcher	2.39	1981	64	25	25	.500	64	234	11	4	0	410.2	349	109	123	30	132	3	0	14	1,685	7	5.13	2.89	1.77	0.66	13.9	7.8	1.17	.248	2.70	7.65
20	Pitcher	2.39	1972	34	8	11	.421	34	128	3	2	0	210.2	157	56	63	16	56	4	0	7	838	3	5.47	2.39	2.29	0.68	15.3	6.7	1.01	.222	2.69	6.71
21	Pitcher	2.39	1931	14	7	7	.500	14	75	7	2	0	109.0	91	29	34	5	31	0	0	1	442	3	6.19	2.56	2.42	0.41	17.0	7.0	1.12	.260	2.81	7.51
22	Pitcher	2.41	1914	8	3	4	.429	8	40	5	1	0	67.1	50	18	21	1	22	1	0	2	263	1	5.35	2.94	1.82	0.13	15.2	8.4	1.07	.246	2.81	6.68
23	Pitcher	2.45	1983	26	11	13	.458	26	117	5	2	0	169.0	136	46	54	18	47	7	0	4	681	5	6.23	2.50	2.49	0.96	17.2	6.9	1.08	.240	2.88	7.24
24	Pitcher	2.48	1916	10	5	5	.500	10	32	4	0	0	83.1	62	23	31	3	29	2	0	2	335	3	3.46	3.13	1.10	0.32	9.6	8.7	1.09	.219	3.35	6.70
25	Pitcher	2.53	1917	12	5	5	.500	12	39	7	2	0	81.2	78	23	27	2	12	2	0	0	329	1	4.30	1.32	3.25	0.22	11.9	3.6	1.10	.277	2.98	8.60