Tim Keefe has the best ERA in a season, with an ERA of 0.86 in his rookie season in 1880.

Interpreted as:

least era with a minimum of 1 IP per team game in a single season by player

	NAME	ERA	SEASON	TM	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	ERA-	FIP	FIP-	H/9	PRAA
1	Tim Keefe T. Keefe	0.86	1880	TRO	12	6	6	.500	12	39	12	0	0	105.0	68	10	27	0	16		0		397	1	3.34	1.37	2.44	0.00	9.8	4.0	0.80		2.31	35	2.25	91	5.83	3.6
2	Dutch Leonard D. Leonard	0.96	1914	BSA	36	19	5	.792	25	176	17	7	3	224.2	139	24	34	3	60	8	8		842	2	7.05	2.40	2.93	0.12	20.9	7.1	0.89	.229	1.36	36	1.95	70	5.57	20.4
3	Dan Casey D. Casey	1.00	1884	WIL	2	1	1	.500	2	10	2	0	0	18.0	23	2	17	0	4		0		83	0	5.00	2.00	2.50	0.00	12.0	4.8	1.50		8.50	32	2.83	104	11.50	-0.1
4	Ed Cushman E. Cushman	1.00	1884	MIL	4	4	0	1.000	4	47	4	2	0	36.0	10	4	4	0	3		0		125	1	11.75	0.75	15.67	0.00	37.6	2.4	0.36		1.00	8	0.91	8	2.50	17.4
5	Mordecai Brown M. Brown	1.04	1906	CHC	36	26	6	.813	32	144	27	9	3	277.1	198	32	56	1	61	4	4		1,044	2	4.67	1.98	2.36	0.03	13.8	5.8	0.93	.236	1.82	39	2.08	75	6.43	21.1
6	Bob Gibson B. Gibson	1.12	1968	STL	34	22	9	.710	34	268	28	13	0	304.2	198	38	49	11	62	7	0	6	1,161	4	7.92	1.83	4.32	0.32	23.1	5.3	0.85	.230	1.45	38	1.77	63	5.85	34.8
7	Christy Mathewson C. Mathewson	1.14	1909	NYG	37	25	6	.806	33	149	26	8	2	275.1	192	35	57	2	36	0	4		998	4	4.87	1.18	4.14	0.07	14.9	3.6	0.83	.234	1.86	44	1.62	61	6.28	31.7
8	Walter Johnson W. Johnson	1.14	1913	WSH	48	36	7	.837	36	243	29	11	2	346.0	232	44	56	9	38	9	10		1,271	2	6.32	0.99	6.39	0.23	19.1	3.0	0.78	.229	1.46	39	1.90	63	6.03	42.3
9	Jack Pfiester J. Pfiester	1.15	1907	CHC	30	14	9	.609	22	90	13	3	0	195.0	143	25	61	1	48	5	5		745	2	4.15	2.22	1.88	0.05	12.1	6.4	0.98	.236	2.82	47	2.14	81	6.60	10.6
10	Addie Joss A. Joss	1.16	1908	CLE	42	24	11	.686	35	130	29	9	2	325.0	232	42	77	2	30	2	6		1,209	6	3.60	0.83	4.33	0.06	10.8	2.5	0.81	.220	2.13	49	1.68	76	6.42	19.3
11	Carl Lundgren C. Lundgren	1.17	1907	CHC	28	18	7	.720	25	84	21	7	0	207.0	130	27	42	0	92	2	3		795	2	3.65	4.00	0.91	0.00	10.6	11.6	1.07	.211	1.83	48	2.73	104	5.65	-2.3
12	Denny Driscoll D. Driscoll	1.21	1882	PIT	23	13	9	.591	23	59	23	0	0	201.0	162	27	73	0	12		0		799	7	2.64	0.54	4.92	0.00	7.4	1.5	0.87		3.27	45	2.36	84	7.25	10.2
13	Grover Alexander G. Alexander	1.22	1915	PHN	49	31	10	.756	42	241	36	12	3	376.1	253	51	86	3	64	10	7		1,435	2	5.76	1.53	3.77	0.07	16.8	4.5	0.84	.224	2.06	43	1.82	63	6.05	44.1
14	George Bradley G. Bradley	1.23	1876	STL	64	45	19	.703	64	103	63	16	0	573.0	470	78	229	3	38		0		2,269	34	1.62	0.60	2.71	0.05	4.5	1.7	0.89		3.60	58	2.14	101	7.38	-20.7
15	Cy Young C. Young	1.26	1908	BOS	36	21	11	.656	33	150	30	3	2	299.0	230	42	68	1	37	1	3		1,116	4	4.52	1.11	4.05	0.03	13.4	3.3	0.89	.247	2.05	52	1.52	67	6.92	25.4
16	Ed Walsh E. Walsh	1.27	1910	CWS	45	18	20	.474	36	258	33	7	5	369.2	242	52	87	5	61	4	7		1,359	6	6.28	1.49	4.23	0.12	19.0	4.5	0.82	.230	2.12	52	1.63	66	5.89	34.8
17	Walter Johnson W. Johnson	1.27	1918	WSH	39	23	13	.639	29	162	29	8	3	326.0	241	46	71	2	70	8	10		1,261	8	4.47	1.93	2.31	0.06	12.8	5.6	0.95	.235	1.96	47	2.04	74	6.65	25.9
18	Christy Mathewson C. Mathewson	1.28	1905	NYG	43	31	9	.775	37	206	32	8	3	338.2	252	48	85	4	64	1	6		1,294	6	5.47	1.70	3.22	0.11	15.9	4.9	0.93	.243	2.26	42	2.03	69	6.70	34.9
19	Max Fried M. Fried	1.29	2025	NYY	10	6	0	1.000	10	60	0	0	0	62.2	43	9	14	3	15	4	0	0	251	3	8.62	2.15	4.00	0.43	23.9	6.0	0.93	.237	2.01	33	2.71	65	6.18	10.2
20	Jack Taylor J. Taylor	1.29	1902	CHO	37	23	11	.676	34	88	34	8	1	333.2	273	48	86	2	45	12	3		1,278	10	2.37	1.21	1.96	0.05	6.9	3.5	0.95	.240	2.32	49	2.73	98	7.36	0.6
21	Guy Hecker G. Hecker	1.30	1882	LOU	13	6	6	.500	11	33	10	0	0	104.0	75	15	49	0	5		1		403	7	2.86	0.43	6.60	0.00	8.2	1.2	0.77		4.24	49	2.27	82	6.49	5.1
22	Jack Coombs J. Coombs	1.30	1910	PHA	45	31	9	.775	38	224	35	13	1	353.0	248	51	74	0	115	7	6		1,353	10	5.71	2.93	1.95	0.00	16.6	8.5	1.03	.246	1.89	53	2.09	83	6.32	15.8
23	Mordecai Brown M. Brown	1.31	1909	CHC	50	27	9	.750	34	172	32	8	7	342.2	246	50	78	1	53	7	15		1,280	9	4.52	1.39	3.25	0.03	13.4	4.1	0.87	.234	2.05	51	1.78	68	6.46	32.4
24	Walter Johnson W. Johnson	1.36	1910	WSH	45	25	17	.595	42	313	38	8	1	370.0	262	56	92	1	76	13	3		1,367	21	7.61	1.85	4.12	0.02	22.9	5.6	0.91	.271	2.24	56	1.39	56	6.37	45.2
25	George Bradley G. Bradley	1.38	1880	PRO	28	13	8	.619	20	54	16	4	2	196.0	158	30	66	2	6		7		757	7	2.48	0.28	9.00	0.09	7.1	0.8	0.84		3.03	60	2.21	96	7.26	0.6